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A miter joint would be perfectly stong, cutting/jigging them is the only slightly tempermental part of it. I use miter joint for all but angle iron. The other option would be to cut one channel square and notch the corners off of the other one (somewhat like you would do with angle iron) then weld.
I'm not a welder but I have worked with acrylic and lexan. You may have already gone here, but JIC: Be sure you allow for the thermal expansion/contraction of the panels when you design the frames. I don't recall the specs but the size change _has_ to be taken into account with the plastics otherwise there will be problems. I've seen this first hand on boats when the owners "reglazed" and did too "perfect" a fit up - cracks big time and bowing.
My other caveat regards planning for water. I'm a plumber and I've spent 25 years looking at the pitiable results of water getting where it's not supposed to. Make sure the frames can drain and don't hold a puddle at the bottom. It's way better to have a drip you don't want and know you have a problem to fix than to live in bliss for a year or two and have the thing sag because the bottom is rusted out. Besides in most out buildings, some water is expected on the floor; you just don't want it to accumulate in places you don't know about. And do NOT depend on caulking of any kind whatsoever to keep water out - won't work for more than a couple years max even with lady luck loving you. If you want to keep water out you have to use good flashing of the right metal (to live long and avoid galvanic corrosion) installed correctly. Flashing is not rocket science but if you don't do it all the time it needs careful thought; lots of good stuff to be found on the net. The basic idea is "think like a droplet of water" - if I were _here_ where would I go to 1) get lower (gravity) or 2) get away from the wind. Water also will ooze horizontal and even uphill a slight distance (3/8") if it has a chance so at the point it drops off (hopefully) you need a "drip edge" hanging down more than 3/8". Just like on a car body, any seams it gets into it will stay there a long time and work on the metal.There is also a great deal of splash back against walls if water (rain) hits the ground within about 2-1/2' so the first 24" above ground level needs to live through drenching and spattering with dirt (which delays drying considerably).
It's great to be moving on your Plans. <g> Best luck.