I don't have plans myself, but if all you need are dimensions, I believe I've seen some online. I'm working with a woodworker friend of mine to build some benches with some small cold-frame type greenhouses sometime, once we both have time for a project. Coming up with the plans was surprisingly easy; just brace the stress points with 4x4s and include cross-braces to keep it from collapsing. The benches we've planned should be able to support about a half a ton each (although I wonder how precise that value really is.)
You can get good corrugated polycarbonate around here at like $2 and change for a 20 square foot sheet.