Here's a goofy little diagram of a self-harvesting black soldier fly larvae composter that I described, in case anyone wants to get an idea of what I'm talking about:

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Note that you may want to use the bucket's lid - just cut a hole in it so that there's an exit point around the "ladder" which the larvae will use to get out of the bucket from. BSF larvae will outcompete just about anything else and they seem to emit a pheromone or something that deters other critters like flies once they've colonised a patch so once it's going you're unlikely to have any problems from small pests.

Scrap wood is ideal for making the "ladder" with but you can use whatever material you have lying around that is going to survive the conditions and which is rough or textured enough that the larvae will be able to cling on to it to crawl out.

The collection container is optional but just make sure that it's tall enough that if you get a large number of BSF larvae collected that they won't be able to crawl out the top. (Although it's probably not the worst idea to let a few larvae pupate so you're putting some BSF back into your local environment rather than trapping all the eggs and harvesting the larvae. It's hard to imagine making a noticeable dent in the local BSF larvae population with this given how prolifically they breed but you never know.)

I forgot to mention that ideally you should put a BSF egg trap directly above the compost. BSF love to lay their eggs in tight crevices so without a trap it's not uncommon that they'll lay eggs around the rim of the bucket if the lid is on but you can maximise your chances easily with about two minutes of extra labour.

The simplest option is to get something that is corrugated like cardboard or corrugated signboard (y'know the plastic stuff that has the same structural pattern as corrugated cardboard). Cardboard won't last forever so you'll have to replace it occasionally if you use that as the material. You can cut a few pieces and stick them together if you like, or just use one piece and affix it to the top point of the bucket on the opposite point to the "ladder", near the rim of the bucket. Don't forget to cut a small hole in the top of the lid so that BSF are enticed to enter near the trap to lay their eggs. You could also use 1" wide strips and stick them on the upper half of the inside of the bucket, using as many strips as you like (remembering that you must cut the material the right direction so that you're maximising the number of exposed holes).

Some people take corrugated material and roll it which is fine but you have to be careful not to crush too many corrugated channels and tbh it's a bit overcomplicated. A few flat layers of corrugated material fixed together will work just as well but even using a few layers is probably overkill for a small setup like this bucket.

If you wanted something more permanent and less plastic-y, you could get some scrap wood and drill holes in it on all exposed surfaces to make something resembling honeycomb using a drill bit which is roughly the same width as the holes in cardboard corrugations. Personally I couldn't be fucked doing something like that but if you enjoy woodwork and you're killing time then go nuts.

Also it's worth mentioning that most BSF bins for sale are ridiculously pricey, overcomplicated, and not necessarily any more effective than a bucket with a few holes, a bit of wood, and a bit of cardboard at the top. They are the worm farms of the BSF world - expensive pieces of plastic that are designed to make the whole thing seem more complex than by rights it should be so that consumers are convinced they need to buy some purpose-designed crap that may require significant intervention to maintain.

BSF larvae happily colonise piles of cow shit without any outside intervention, just like how compost worms will colonise a pile of decaying vegetable matter without needing any help. Don't let the marketing convince you otherwise.