I wanted to use my Eggtimer Quantum in smaller rockets and needed a way to secure it and a small 2S LiPo battery safely.  I found the Arduino Nano holder by HD_Creator was nearly the same fit, so I downloaded it and used it as a reference for the Eggtimer Chassis.  I built it so that there is just enough room below to house the battery and modeled grooves for the quantum to slide in.  There is an arrow embossed on the bottom showing the direction to load the quantum with a positive stop.

It's not perfect but it solved the immediate need to get something around the flight computer for launch.  It's a bit larger than I intended.  I was looking to get it into a BT-55 body tube but it's the ol' square peg in a round hole problem.  I ended up using it as part of a more complex avionics bay for a 3-in body tube and it works really well in that configuration (I will link to that model when I am done with it).

One issue is that the sides block the connectors for deployment charges.  This can be fixed by reversing the connectors so they face inwards.  Since I am not using this for deployment, only for altitude, it wasn't an issue for me.

Feel Free to use or modify as you see fit.  I would love to see what you do with it.