Look around and find some high density styrofoam float logs. They use them to build floating docks. they are generally 12"X16" x 9' long. I've been using the same two for over 5 months now.

I attempted to build a butt out of the sill seal foam like the block targets. Talk about a P.I.T.A. Harder than hell to stack and after spending 3 hours buiding it my arrows went right through it like butter.