I am nearly finished getting my rain barrel system up and running. On the 'need to do' list is add mosquito netting, raise the barrel up on blocks and install the gutter system. On the 'I'd like to do' list I have a water level indicator and secondary barrels. If a really good solution presents it's self for a water level indicator I will probably add one of those.
I'm always on the lookout for a free solution to a problem. I had a spare bed skirt that was headed to the trash, before tossing it I evaluated it for possible use as a mosquito net. It seems to be fairly tightly woven, which would keep out mosquitoes but not too tight as to keep out water. I think it'll work fairly well. I cut out a section to fit over the hole in the top. Using silicone directly between the barrel and the fabric I laid it out. I cut the section large enough to leave a decent amount of slack. by pressing down slightly on the fabric against the barrel the silicon seems to permeate the fabric which I hope will provide a good seal. This solution is cheap enough that if I need to get back inside the barrel (like for adding a water level indicator) I can tear it out without feeling like I wasted money.