I think that you have answered your own question by closing the vent securely. It looks as though they will be just fine for now.
I certainly recommend getting a cage off the floor for several reasons: 1) It's always more drafty on the floor, 2) your cavies will be more comfortable if they are elevated.....you don't look like such a giant house eagle hovering over them, and 3) It will be much easier on your back to interact with them and to clean the cage. Plus, by putting the cage on a table or making a grid stand for the cage, you can store all things piggy in canvas or plastic bins beneath the cage.
As an aside, your cages appear a bit crowded. I would rearrange them so as to concentrate food bowls, water, litter box, etc in one cage, and leave the other relatively open so that they can have room to run.
I know that it is recommended, but do you really need two of everything? My boys share a pellet bowl, water bottle and litter box without a problem and it reduces crowding in the cage.
Wait a minute........I just noticed three cavies? Perhaps you do need that much! Your cage may be o.k. for sows if they get along, but appears much too small for three boars. They need a lot of space, especially as they grow up.