Summary

There are lots of coin traps on Thingiverse, but I haven't seen any designs that embed one trap inside another. Here's how to do it (assuming your 3D printer is capable of pausing a print in progress, which not all of them are):

1. Begin the print (I used no raft, no support, although you can add one or both if you must).
2. When the smaller coin trap is about two-thirds printed, pause the printer and drop a coin into the center. A Canadian or American 10-cent piece fits very nicely. Be sure that the coin does not protrude above the printed area.
3. Continue the print. When the smaller coin trap is completed (with the coin trapped inside), pause the printer again. Remove the smaller coin trap from the print bed and drop it into the larger coin trap, again ensuring that the smaller (inner) object does not protrude higher than the side walls of the larger (outer) object.
4. Unpause the print one last time.

Notes
a) Try rotating the completed object on a flat surface. On my test print, the inner (spheroid) object always aligns itself with the outer (cubic) object, which looks pretty cool.
b) These objects were made with Meshmixer 3.0.