keito, what type of hive stands are you using? Does your screened bb sit "elevated" above your hive stand by the rails on each side of the bottom board? This might form a ledge that would hold a sheet of some thin (plywood, plastic, hardboard, cardboard, etc) material. Those ugly election signs made from coroplast are popping up everywhere and will be until after the election and they make great stickyboards, panels to seal off screened bottom boards, etc.,.they sure do clutter up the roadside, too! 8-)
Actually, the seal at the bottom doesn't have to be perfect...a little gap here and there would probably help with ventilation/moisture. But, if you use something thin like coroplast and you wanted a better seal you could glue a strip of .25 square wood across each end of the coroplast to help cut reduce wind from blowing in...slip the coroplast in with the wood strip on the bottom...you could put rails down the side of the coroplast as for as that goes to make a very rigid insert. I think just a piece without the wood would work good, though.
Here's a shot of what I'm talking about regarding the space between the bottom board and hive stand. This is a shot where I'm experimenting with a roughly made oil tray...