I use these LEDS year round for various holiday displays, but the largest is Christmas displays. When I discovered this project I was estatic as I already had thousands of these LED 'pixels' and a several Raspberry Pi's  on hand. For a typical Christmas "Mega Tree", we use pixel strips to make each leg of the tree.

 I used a thin version of the strip to hold each LED in the same position. If you look at one of the 'bullet pixels', there's a mounting notch as seen here:  

 This is where I snapped the pixel strip into place and then stapled each one to the back of the thin plywood backing.

 These strips are VERY inexpensive and can be purchased here: https://boscoyostudio.com/product/the-original-12mm-pixel-rgb-mounting-strips/

Closeup of them stapled in place:

This ensure each pixel is perfectly aligned on the Z axis and they're not going to move. No need for any glue. 

Also, there are many pixel vendors I work with that will do custom string lengths and distances. I can order 100 pixel strings with 6 inch spacing for about 25-30 dollars. PM me if that is reasonable. Sure makes sense not having to run null pixels because of the standard 4 inch spacing.

Also if you weren't aware, these pixels do come in a square form factor and that would allow us to use shallower frames.

Cheers!

Steve