TPU is the perfect material for approximating a PopSocket. With 7.3g of filament you can print your own and save a few bucks.

This design is about 2mm taller than an actual PopSocket when compressed. I have included my Fusion design file if you think you can improve it.

Printing: 0.15mm layer height (at 0.2mm it doesn't stay compressed), 15% gyroid infill, 2 perimeters, no supports. There is long bridging on the top which can result in cosmetic “warts” on the finished product. If this happens just try printing another one. For the most part mine have been wart free.

Material: I have only tested Teal and Red Sainsmart TPU. I'm rooting for all of the other TPUs out there but I simply don't know if they will work. Not working means that it doesn't stay collapsed.

Here are some inexpensive adhesive circles for the bottom.