Dave don't be fooled into thinking profile is flat or level because it often isn't and to make this worse your design makes it so your pretty much guaranteed you'll have issues with the rails binding.? . . .Your relying completely on the quality of cut on those uprights to keep the rails aligned and 90deg to the base. The 90Deg brackets are not accurate enough or strong enough to hold everything square. The fact your using 45x90 orientated in it's narrow section when the Y axis forces will be trying to rock the profile doesn't help and this is compounded by the fact the 90deg brackets don't hold it in that direction either.

Sorry to say this but while you think the frame is strong it's actually weaker than you think because it's not braced or held the directions that matter and your under sizing the profile. To make profile work for this design you'll need 90x90 for all the base and much more bracing in correct locations using plates not those poxy 90deg brackets.

For this reason Steel is much cheaper, stronger and more accurate if used with the epoxy method.