Dave thanks for being patient on the multiple attempts to make your custom grid fit correctly and for writing this post.
The Velcro straps on the outside of the grid made a slightly smaller grid work (possible halo?) but for your custom grid it would just fall out. I wasn't aware it was slightly smaller. I'm glad you brought it to my attention so the grid could be improved.
Shoot, I should change the Maxilite Grid product photo on the website because the thinner outside rim with Velcro straps looks different.
Side note: I've had trouble sourcing the smaller 1/4" - 3/8" channel that makes the outside rim of your grid. For your grid I ripped the 1/2" channel down to 3/8" on a radial saw. So, unfortunately it's not practical for production (a little dangerous and scary, too).