Susiebell,
I was going to do a similar thing but on a wood frame. Did it last year and all the plastic got blown to bits!
So this year I am going for corrugated plastic roof, (£6.00 a sheet from Focus)which I am going to screw onto the frame, could you not do the same but with bolts?
The sides of mine will be plastic again, but this time I am going to put batons over the top of the plastic (from pallets) for more protection against wind damage!
DP