Happy to help!
I use a standard reptile lid on top of my 20 or 10 gallon, but I use like 20lb fishing line or floral wire(depending on the type of environment 'wet' vs 'dry'), and stitch a fine window screen to the top.. This will prevent all but the smallest of insects to escape thru the top. You can then use a strip of draft foamtape to run around the top of the tank, as a seal for the edges.. I've felt the need to improve on this part, but it should give you some ideas to mull over. Been a while since I've built one, but I think I have one left around somewhere.. My son was unstitching them because he didn't need the fine screen. I'll look when I'm out in the garage again. I think I've illustrated it well though, no?
Then, if you want to get even fancier.. You can cut the top off a milk gallon, leaving enough flare to sew it to the top of your cover (or fuse it with hot glue, etc.). After you are sure it's fused well, you can cut out both screens from the center of it, making a small drop port you can open and close without having to remove the whole lid. You will find a number of containers that will fit snuggly into this system..
Like M&M cylinders.. Those little cheap plastic yogurt slurp (blah!) things that are banned from here.. Hoses.. Etc. The possibilities are as endless as your imagination.
I also use this method for making ports and vents on Sterlite type containers for arthropods. Never a shortage of milk gallons around. I usually drill a hole that the cap will just barely not fit thru, so I can use the ring to lock it in place.. Sometimes I crack the container.. If I rush it. Don't rush it! You can also use these as small ports into bait capture traps, swapping out a lid w/holes for one that doesn't have one, when you want to collect.. For a very passive/lazy method.
Not sure what you meant by keeping them alive.. I don't kill many of the fruit flies using that water method, but I don't clap my hand hard either, allowing the water to do the work. Then you just carefully wipe them on an object in the tank.. They will usually recover. I was feeding them to Ladybugs, so I just stuck them on the honey-dipped toothpick and served them up like candy apples..
And yummy delicious winter mosquitoes, for variety..
I used this base for rearing fruit flies, and other stuff.. Like an aquarium it adapts nicely to many varieties of culture.
Hope that gives you some ideas!