I recently bought a reel of Ninjaflex TPE filament and wanted to test it on something useful. I need to carry two different RFID keyfobs for my work and I hate cluttering up my keyring, so here is my solution. The ribs are there so it can be stretched over my hand then shrink back to wrist size. On my print the RFID tags are very firmly held and will not fall out. This wont work in PLA or ABS, you definitely need a flexible filament. This could easily be adapted to hold a keyless car fob, USB drives, bullets or even a small pill bottle like this: http://www.thingiverse.com/thing:19240. Freecad file is included so you can tweak it to suit.

Ninjaflex is absolutely beautiful to print with. It sticks well to an un-heated bed and has awesome layer to layer fusion. It works best with a direct extruder rather than a Bowden and the filament needs to be guided to the hot end from right up close to the drive wheel. My setup uses a Wade extruder with a Budaschnozzle and it works great. I definitely recommend using Cura rather than Slic3r for preparing G-Code, Cura does no perimeter departures on this print so it leaves no pimples on the vertical surfaces (Ninjaflex is elastic and a bit more prone to oozing).

This particular thing is definitely not a one size fits all. If you try to print it, I'd suggest doing a test print and stopping it after a few mm of build to try it on for size first. Bear in mind that the full print will not be as stretchy, so if it is tight to fit when only 3mm thick, it will be a real struggle when fully printed. The Freecad file is included if you need to change the size. If you edit it, make small changes and save often - the sketch is not fully constrained and you might need to re-open the file occasionally to get back to a stable model.

I have challenged Logan to code an OpenSCAD version of this for customiser so keep an eye out for it if Freecad isn't your cup of tea:
http://www.thingiverse.com/logan12358/made

I don't represent Fenner Drives, I am just a very satisfied customer happy to endorse an excellent product. I'll definitely use flexible filament where I need something that is super tough and that has a bit of flex or grip to it.