I've seen a bunch of valve caps floating around the 3D printing community, even some D6 valve caps. Never seen a D20 that I could screw onto my car tire before tho...

So here you go! D20 modeled individually by me, text is the "Dungeons" font, created by Darrell Flood. Font package can be found here:

https://www.dafont.com/dungeons.font

The license for this font is non-commercial, and therefore so is the license for this model.

Schrader valve stems use a weird thread size, 8V1-32. It must've originally been specified by a sadist (now the specification is in ISO 4570 https://www.iso.org/standard/30078.html), but at least it's a normal 60 degree threadform.

Took a couple test prints to get the threads feeling right. Might be a bit loose or a bit tight depending on what you thread onto. Print at 98% or 99% size for a tighter fit, 101% or 102% for a looser fit.

I tried printing this without supports with the opening of the valve threads against the bed, but there is not a lot of face area there. I spun the model to print with the #1 face against the bed, and it seems to print better. I also included a separate custom support model that can be added in Cura for printing from the #1 face. Definitely need a brim for this one though.

For some reason (probably my poorly calibrated printer) the faces that share an edge with the face on the bed are a bit rough. They're mostly against the rim, so it didn't bother me too much. Been driving around with this little guy (printed in PLA) rolling nat 20 perception for about a week now. No collisions yet!