An improved version of these is available on Makerworld here: https://makerworld.com/en/models/1235398-dnd-coin-weighted-miniature-bases-1in-25mm

Remix of the awesome miniature bases made by Valandar, see here: https://www.thingiverse.com/thing:3067745 https://www.thingiverse.com/thing:3314896

I added a circular cutout in the bottom that will friction-fit a US nickel. The hole in the middle is so that a nickel can be popped back out with the help of a paperclip or SIM removal tool.

Tolerances

On my standard Ender 3 these print with a range from just barely tight enough to hold a nickel to so tight that I have to snap one in with a pair of pliers- but most are just right, and a nickel can be easily snapped in and out without falling out during use.

If these don't print with quite the right fit for you, you can probably just scale an entire base STL file by 1-2% to get a perfect fit on your particular printer. But if you want to properly scale just the hole I've included my blender file (and sorry in advance, it's kind of a mess).

Other

I also included a Nickel Outer Ring to put around existing on-nickel minis so that everything will have the same base diameter.

EDIT 4/2/20 Added Nickel_outer_ring_flat.stl. The original nickel outer ring model was rounded on the top to make it look nicer, but in practice this just led to an unreliable fit. The flat version is the full height of a nickel, snaps on more reliably, and can be glued underneath models that already have a flat base to easily convert them to nickel'ed bases.