Your best lightweight option is to use metal roofing or the corrugated plastic roofing panels. I know you had a bad experience with the latter, but that was purely because you installed it wrong-- of COURSE it blew off that way
Install it CORRECTLY and it will stay put at least as well as anything else will. Use polycarbonate not PVC; or use metal, which is usually a little cheaper and not much heavier. Make sure to learn all of the necessary details about installation and support before doing the job, and it will make an excellent roof.
Or, JUST plywood and shingles is not going to be any much heavier than what you've already got on there. (i.e. remove all that, *then* put on plywood and shingles)
If you will shingle the roof, you can graft on a small roof overhang by replacing your current roof materials (whenever you do that, I mean, not necessarily right now) with plywood that hangs over 6" or so. That amount of unsupported overhang, with at least 1/2" plywood, will be fine. If you wanted more overhang you could add supports that brace against the top of the wall, lined up exactly with where the studs are; but honestly 6" of overhang is all you really need to keep the weather out of your wall structure.
If you are going to use the plastic or metal corrugated roofing panels rather than shingles, you can either use the same method (the plywood needn't cover the whole roof decking as long as you use adequately strong adequately-closely-spaced purlins) or you can build out a little overhang with 2x4s, again bracing them back against where the studs are contained in the wall. You would have a 2x4 running horizontally along the edge of where the roof panel will come to, plus braces attaching it to the upper walls. Heck, if you go with a smaller roof overhang you could pretty much just use good outdoor-grade L braces for it, it won't have to bear *lots* of weight.
Good luck, have fun,
Pat
