At first, since I don't have a 3D printer and a suitable transmitter box yet, I made its body in the most awkward way: wrapped the board with electrical tape (but without covering the battery itself so it can be easily pulled out).

And the receiver body was made of ..... a thread spool! Yes, such a silicon spool can be cleverly cut and turned into a device case. It looks like a well-designed backpack for a turtle. Black stretch bands are fixed in the case with metal braces, and their length can be adjusted to suit a specific turtle.