Somebody was offering similar voltage testers printed in bright orange color few years back, but I couldn't find that thread. (I found only these: 15341, 17675, 24373)
Putting them on a keychain or lanyard, as @antichamber suggests, would actually make it very convenient and so much harder to forget at the pits.
@antichamber, if you are going to design and 3D print a custom case, may I suggest a couple of modifications for "deluxe" version.
1. You could add a 2-pin dupont female connector to plug in a motor or y-cable. Seeing voltage drop as you run the motors is a good indicator of how much power it consumes. You could estimate the friction loses, output load, etc...
Last year I made a bunch of these and was giving them out as on-the-spot prizes, when I was pit judging. (they were not 3d printed but hot glued and looked ugly)
2. Since you 3D printing, add a couple of holders for 5/64 and 3/32 L-keys and, maybe, even a combo wrench.
3. Add a hole or two to hold spare 6-32 motor screws, an 8-32, as well as pegs for a shaft collar and a nut - a nano toolbox on the go.
4. If the final product could be mounted with a small loop of VEX chain that will look awesome!