I have a Nashbar trailer that I put a solid plastic floor into (drilled holes in it and used zip ties to attach to the frame). Had to put pipe insulation tubing around the edges so that it didn't tear the bottom material. My dog weighs about 45lbs and it works just fine. I also made some other simple modifications to it so that she can lay down.
My advice is to not spend too much money on the trailer and spend the money in the modifications.