Check out the Riga kits. Made in Germany, super sturdy, 10mm twinwall, and very well designed. Costco even sells a version that comes with a ventilation kit.

I know there are others too, but I can only speak from experience about the Rigas.

Agree with Karin, we just finished assembling our Riga IV and are very pleased with the quality. We just installed our heater (not a moment too soon) as we plan to maintain keep cold weather herbs and vegs going through the winter.

I am looking at companies like Rimol, Atlas, and Janderloon for what I believe are the best kits available. 8 mm poly carb. Is actually good value for the $$ spent in my view. R value increases very little with the 10 and 16 mm versions and I don't believe longevity is proportional to the extra cost as well.
Let me know if you feel otherwise, I am always open ears to differing opinion.
But how can you beat a galvanized steel frame made from square tubing? I think lifespan can approach 30 years. Of course the pc won't go that long.