Here's an easier way -- Secret Weapon miniatures has 40mm beveled hollowed blanks -- $10 for 5:
http://www.secretweaponminiatures.com/index.php?main_page=product_info&cPath=79_80&products_id=338
1. Roll (flatten) a disc of green stuff > 40mm, but relatively thin (about 1.5 - 2
mm). Press the
GS disc into the 40mm base, and clean up the edges. Flatten the
GS so that it's a nice working surface (it doesn't have to be perfect).
2. Press a blank, slotted 25mm base wherever you want your 25mm base to sit. This will cut a donut hole into the
GS. Remove the 25mm base by lifting it from the slot, remove the
GS in the middle, and let the
GS on the base cure. Once it's totally hardened, dress up the 40mm donut.
3. Put a magnet on the 40mm base, and one on the 25mm base, and presto!
I have also seen 25mm bases with straight edges (instead of beveled ones), but I'm not sure where. This avoids the unsightly gap at the top. I suppose they'd be easy enough to cast in resin -- you could just use a stack of quarters to make a mold.