I have wire bottoms on all of my nest boxes - actually removed the original peg board and replaced with wire. Solid bottoms were getting too dirty and since we used the pegboard, it was really absorbing moisture.
I put a piece of cardboard in the bottom - cut to size - in the winter. Then straw on top. I pull the cardboard after 8 to 10 days because it, too, starts absorbing all the moisture. The cardboard comes out along with everything else in there. I just put some hay back in the bottom now. The hay and wire bottom lets everything fall through that needs to.
I agree with 3mina that you have to be careful when you move the box - lift rather than drag. Usually, the kits are on the hay and not directly on the wire bottom.
Warmer weather, I don't put the cardboard in. Stuff can and does fall through but it's cooler for the kits.
Hope that helps.