Homemade insecticides usually just manage to make pests laugh. Unless it's nicotine-based, and nobody really wants to handle that stuff anymore.
If you are going to use netting it needs to be very fine. Once upon a time, before insecticides and refrigerators became widespread, there were two/three storied containers with a wooden frame and covered in very fine netting to keep flies away from food. If you can find or build one of those things it's the ideal stuff to keep pests away from seedlings and tender green vegetables.
If you want to use Bacillus thuringiensis (BT) var aizawai or kurstaaki, Sumitomo sells it in Europe under the Xentari brand name. I advise against buying smaller sized products because they are sold at extortionate prices. Intriguingly enough the israelensis variety, which is perfect to kill tiger mosquito larvae, has become impossible to obtain in Europe and I have to order the stuff directly from the US. And in the present situation it may mean months of delay and a huge shipping bill.
