I have a high-quality Cyclus reamer for STs of the standard (for me) size and a very inexpensive adjustable reamer like the ones in these pictures that I used for an unusual size (around 32mm). Was probably one of these:
https://www.ebay.co.uk/itm/284244191...wAAMXQ0pNQ-7YD
or similar. Made in India. Works absolutely fine, although maybe it won't stay sharp for as long, but then I don't use it often. The cutters look like they're made of high-speed steel. Just scooch it round a few times, wind it out half a turn, and repeat until the seatpost fits nicely.