Could you makes one for Rock Pi 4, needs some adjustments because of the heat sink.
The Rock Pi 4 has the same dimensions as the Raspberry Pi 3 which should change are the anchor points, in this version for the Raspyberry has 2 anchoring plates that are on the bottom of the plate.
It would be necessary to change the position of these 2 points and add the other 2 anchor points, this is neccessary because the Rock Pi 4 has a quande heatsink at the bottom.
Please enter the website of the Radxa wiki and look for (3D drawing STP) you will find a 3d model of the board and I recommend looking for the photos of the board with the big heatsink so you can see what is necessary to modify in your model.
After the modification this model will remain compatible with the Raspberry Pi 3 the change are few.
thanks for listening.