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Forget the springs for the belts. That will just create issues that will look like wobble every time the head changes directions suddenly.

For stiffness and squaring the frame use the stapler on the lower corners:

I still haven't built mine, but that because I'm getting all the parts together first. Go to Open Builds and buy the metal spacers for the wheels on the gantry plates. Those nylon spacers are garbage.

I think they all use the 9mm spacers. If you measure the stock nylon ones they come in around 8.5mm, unless they were over tightened. Speaking of that, don't over tighten the wheels...The bearings and shims in these wheels are junk too. If you over tighten them they will lock up. Tighten the wheel until it just starts feel like it drags when you spin it. It only has to be tight enough to not move around when the gantry is installed (no shifting or rocking in the gantry). Beyond that, just pay attention to what you're doing and you should be fine.

2020 Corner Bracket