Ok so I know there are pre made solutions, but can you pack a 800mw 30x30 VTX in that? No ya can't :D So this again for my buddy with an RC car, idea was to be able to get him into FPV with out flying lol. I have used Panda 30x30 and Cyclone 30x30 VTX's for this, as long as they have a 5v out you can wire the camera to, and a 14mm camera. The square spot on the top is to fit a raspberry pi heatsink onto, and the round hole is for the antenna to come thru, easy to move around in the tinkercad link below if needed. I have it setup to have a XT30 mounted out the side with hot glue to hold it in place. I tried just having a pigtail out the side but this is better. The 2 slots on the back are for Velcro to go thru to hold your battery, that will depend on the VTX you use, but the ones I am using are 2-6S so makes it easy to power. Then stick some Velcro, or VHB on the bottom and go to town. Here is the tinkercad link if you need to mod it to fit another size camera or something. I was also thinking of doing a 20x20 version.
https://www.tinkercad.com/things/lCkstBx4S3z-vtx-backpack/edit