Earlier in this thread there was a discussion about using HDPE or equivalent to side plates or other parts of the structure. Have you thought of using this as it has a inherent amount of flex that alloy does not. I spoke to 2 different suppliers in regards to HDPE and both stated that UV (one of the issues) would effect the product but most likely not in the application like this. The guy that we used to water jet cut the plates we made would be able to cut HDPE as well, and as a side effect its also easy to mod in your own shed. From a strength/weight perspective it can be made about 3 times thicker. Which depending on the alloy used is about ~12mm (1/2 inch) which is a pretty solid bit.
Check about 30 seconds in. If you've seen this already then apologies for bringing it up again.