A set of passive radiators for all your speaker design ideas.
I've included a few premade sizes in both circular and rectangular, and parametric Fusion 360 archives so you can roll your own. Also, STEP files.

Passive radiators function like bass reflex ports, but allow you to tune small boxes to lower frequencies by adding mass to the radiator. Getting predictable performance requires careful design and modeling. However, if you're just having fun, print one up, stick it in a box and add mass to adjust the tuning. This is an example of what can be achieved if you do it carefully:
https://www.thingiverse.com/thing:4825181
Sound demo here:
https://youtu.be/pLwOpjRZ1ew

If you like it, please hit like!

The surrounds need to be printed with a flexible filament, TPU, Ninjaflex, TPE, etc. When printing the surround, turn on "detect thin walls", and/or set extrusion width to 0.4mm.
The double membrane designs will take more weight without sagging, and have perfect symmetry in the surrounds, but using the single membrane versions will let you tune lower with less mass. When using the single membrane versions, try to keep the center of mass as close as possible to the plane of the membrane, to prevent it sagging. I use Gorilla Glue Clear to glue the surrounds on.