Here is a case I made for the TrimUI Brick Hammer. I made another similar case for the Retroid Pocket Classic. This and the RPC model are adapted from the amazing parametric magnetic storage box that @Mihai_B_Mike made, sized down for the TrimUI Brick Hammer, and then I made some custom inserts to protect the handheld while in the case. 

This should work just fine for the TrimUI Brick (non-Hammer) version as well, as they are essentially the same handheld. The inserts in this one are a lot more optimized than my RPC inserts, except one problem with this version is that the spot to put your finger to pull out the handheld is a bit small. If you make the inserts out of TPU (which I recommend), it's at least easier because there is a bit of give with the material. 

Based on the parametric box, you will need 2 - M3 x 30 screws to secure the hinge, and 4 - 8x3mm magnets. The magnets are really more or less a press fit and I haven't glued them in. In fact, with this one as opposed to the RPC version, these magnets were much harder to get in, but if you press hard enough, it'll go. Might just take a bit of elbow grease!