The Pygmy avionics bay is designed based on this video by Matt Pritchard, as well as numerous other similar bay designs I have seen online and during my time participating in IREC rocketry competitions.
Here is the Pygmy within my L1 rocket:
It is a convenient design that makes removing the avionics bay and changing batteries relatively easy. It is also pretty simple design-wise to customize the mounting plate to meet the needs of your avionics system setup. Mine uses a pull-pin switch since my university rocketry team has always used screw switches, and I have always wanted to use my "remove before flight" tags properly. Yours may go a different arming route, or have COTS components for redundancy.
The design was made in FreeCAD and I have included the parameterized design files to make it easier to modify.
This design was made to be 3D printed and used with heat-set inserts, then mounted on the rocket bulkhead using two threaded rods. All screws are M3. The battery case in this design was made for a single-cell 18650 Li-Ion, but you should be able to change the battery dimensions to fit a dual-cell or something different like a LiPo.