*pulls out rulebooks/pdfs*
Compression works.
I think the ring is actually in Lords of Madness, under the Tsochari section.
Getting it to activate a Rod of Wonder a whole bunch of times might also work, if your DM's the silly type.
Sadly, the Ring of Reduction in Lords of Madness only works on intelligent creatures (which, without an
awaken construct spell or similar effect, a flesh golem is not).
The ring of Reduce Person in Savage Species should work, I think. Many magic items count the user as the caster, and you always ignore your own spell resistance when casting a spell, which would include ignoring magic immunity, since it is infinite SR. The Ring also works on pretty much anything (it's not actually called Ring of Reduce Person... I just can't remember the name...). The psionic power Compression should work also, if you can get it into a magic item the golem can use.
The ring of Shrinking (SS ring) is once a day with a 9-min duration. Reverse-engineered, a custom version of the ring without the use/day limitation would theoretically have a market value of 18,000 gp. Interestingly, a continuous item of
compression (manifester level 1) would only cost 12,000 gp.
*closes rulebooks/pdfs*
... Now back to your regularly scheduled mecha-ogling