I have a 3d printer that hadn't been used in over a year. After a brief round of troubleshooting a bad motor driver, I was actually able to design and print some brackets for the LED board without much trouble. My design tool of choice is Designspark Mechanical (it's a stripped down but free version of Spaceclaim, which is amazing.)

I started with a basic bracket to join the four sets of inner corners. The cutout is to get around an LED that is a tad close to the corner mounting holes.

I then made models of the frame itself (a shadow box that was on sale) to help with the edge brackets. (2nd rev shown)

The brackets bolted up nicely with one adjustment to the hole diameters

But unfortunately the brackets did not make up the gap to the edge of the frame. (Gap on right here)

The prototype partition grid fits well, but still isn't even enough for the final product.

And finally, the rev 2 of the edge brackets, which should be easier to mount and should take up the remaining gap.

2 of these need to have the cutout on the long edge, and two on the short edge. The remaining 4 don't need a cutout, but I'll use ones with a cutout anyway to reduce the versions needed.