Not knowing how large of a top you are making not sure if clamps would be possible or not, Screws would work fine if you dont mind them being there when you are done, of course you could remove them after the glue sets but then you would have holes. Possibly buy some 1 1/4" screws and screw from the bottom side and dont let them pull through. Is this bench staying out in the weather? If not the you should be able to buy 1 1/2" high density particle board we use for making counter tops, its hard and smooth and you can buy different thicknesses but it won't take weather.
