You can find it in the comments of this post.

I looked at some of the pre-made bins you can get online and have been lightly pondering how I might make one and where I would dig a hole for it.

Honestly I'd advise against buying a purpose-designed bin.

You can have them in-ground if you like, but it's not a necessity.

BSF are attracted to rotting things and they're actually surprisingly common, even in areas where the winters get very cold. Obviously they aren't active when it gets to freezing temperatures but either they lay eggs that go dormant when the winter weather comes on or they recede during the winter months and then rapidly repopulate, moving from the warmer regions into the regions that have just recently defrosted.

If you have a purpose built worm farm, it's worth noting that BSF larvae will outcompete your worms so it can cause a conflict. There are ways to manage this, including physically separating them but as BSF do not tolerate wet environments, you can take advantage of this biological quirk and throw a lot of high moisture stuff into a worm farm or you can even flood it - the worms will manage but the BSF larvae won't.

Also worth mentioning that it seems as though BSF compost/frass is not nearly as rich as a fertiliser compared to what compost worms produce, and my hunch is that this is because BSF are extremely efficient so they don't leave many nutrients behind.

I would need to order eggs too, because our winters are a little too cold for them.

If it's cheap, go for it.

BSF have a remarkable sense of smell. I'm not sure about the science behind it but it's commonly held that BSF larvae emit pheromones that attract soldier flies because once you have a few then suddenly you will find a big influx of soldier flies. My hunch is that it's an evolutionary adaptation which signals to nearby soldier flies that the ideal conditions for BSF larvae have been discovered, so it's like a beacon signalling the soldier flies to converge, breed, and lay eggs right at that particular spot.

You could try taking advantage of this by buying some live BSF larvae from a local reptile specialist store if there's any around. It's not guaranteed to work but it might be a cheaper option.

In all honesty, there's probably BSF around that you don't notice because they aren't a nuisance like blowflies are and unless you know what you're looking for it's easy to assume that little flying insect that flitted past the corner of your field of vision was just a fly or maybe some sort of little wasp.

If you want to go hunting for them, I'd look at any local animal paddocks, especially if there are cows. You are almost certainly going to find BSF in those places.

If you want to lure them, there are a few things that the soldier flies love. Rotting meat is one, which is pretty off-putting for obvious reasons. They also love corn which has gone sour - you could get a jar and add some cornmeal, masa, unpopped popcorn kernels or whatever you have on hand then add enough water for it to soak and get soupy then leave it with the lid on to ripen for however long it takes before it starts smelling really sour. I'd say that you could do a very similar thing with dog or cat kibble, or you could mix corn and kibble together, and it should attract them while not smelling as offensive as last week's roadkill.

When you do open up the jar, I'd try to do it on a warmer day (say about 20°C or more - the hotter the better) with a light breeze. BSF are very strong fliers but if it's really windy they might have trouble finding the source of the enticing odours.

Honestly though, this is probably overkill. If you have a compost heap, especially one that is open air, you probably have some soldier flies lurking around looking for the right place to lay their eggs already.

My first wild sighting of BSF was in an urban environment where there was no agricultural land anywhere nearby. I had an in-ground compost bin with a lid that I would use to reduce my environmental footprint a little. Without any exaggeration, I would open the lid of the compost bin and I'd get soldier flies within seconds. Not swarms of them by any means but typically just one or a few.

If you have any compost or areas where there's a pile of animal shit, especially if it's a bit stinky, you could cut a small piece of corrugated cardboard, affix that to a stake or a stick and plant it so that the cardboard is directly above the pile. If there are any poles or posts near the pile you could also put cardboard strips on them.

It will only take a couple of minutes to put together with scraps that you have lying around - even if you don't manage to lure any BSF this way it's not like it would be any great loss to try your luck.

Up until fairly recently it was commonly held that BSF were too tricky to keep in a greenhouse or that it wasn't possible to do it. More recently I've seen some indoor commercial operations that indicate that it's practical to keep BSF in greenhouses.

My guess is that because the soldier fly itself only lives for like a few days max, people assumed that they were dying because of the greenhouse conditions rather than them dying because that's just what BSF do.

If I lived in a climate that had freezing winter temperatures and I had a greenhouse, I'd probably just try my hand at overwintering BSF in the greenhouse by moving a BSF farm into it. It's not like you'd have anything to lose by trying it out. If it worked out, you'd be able to have a steady stream of BSF larvae year-round which would be pretty neat. You'd just want to make sure that you allow a percentage of larvae to pupate to reach the adult stage to keep the egg supply going.