I’m glad you asked.
Support the ends of the top bar to ensure there is no chance of the motors flexing. I found out the hard way that things can be more flexible than they seem. Maybe consider making the rest of the frame the same width, as opposed to just 8’. Square tube is the way to go. I definitely don’t want to do it again but if I ever did it would be square tube.
Might also be a good idea to run a horizontal tube across the back.
Also consider adding triangles for better support.
Consider adding supports for skirting around all four sides. I considered it and decided I didn’t need it, which was wrong (if you want to cut near the edges).
I can recommend using weights and pulleys across the top bar for chain slack, as opposed to bungees. How and where will you mount the pulleys?
I made my top bar adjustable and never have needed to move it and I cut 1/4” and 3/4” frequently with zero problems. Great idea in theory but not necessary in practice. Don’t waste the time and effort on it.
Regarding “triangles”, I’m not that familiar with the “default” frame design. I’m just referring to using triangle supports to make things more structurally sound. In very simplistic terms, if you build a square, its corners do not have as much strength to resist forces applied to them compared to adding an additional support to make a triangle in each corner of the square.