Hey CR, i thought of this a while back when you were talking about the ideas for version 2 but never got around to drawing it out for you.
i know this is your design so feel free to ignore my idea, but i thought of a way you can completely eliminate the need for the fold down legs and the pin.
the idea is to use an aluminum i shaped beam like you were saying instead of the tuning fork but instead have there be two round pegs supporting the mobo tray in either channel and a notch in the top of the rear half of the vertical channel. like this:
if what i mean is not clear enough with those i can do some better drawings. I just did those in like 10 min to illustrate my ideas.
i think it would simplify the design some and would make it look really cool. almost like the mobo was just floating there when it was in the bench position.
feel free to completely ignore this post too, i do not want to impose on your brainchild. :-) you really are doing an amazing job. :-)