Abe wrote:
as well as tapered
conical washers that force the bolts to center in the holes; looks like
maybe something like conical faucet washers might do it; to indeed,
filling the holes with putty before tightening it all up.
That's the right fix because you don't want to modify the holes or put
on an adhesive that you will have a hard time removing later.

Go to the hardware store and find those conical things. They work
great for me. They're not really washers, but more of a conical nut.

The best bet is to buy a seat that has the correct
stuff included. The best seem to be the nylon
bolts and nuts. And the best indeed have a
conical washer on one end and a conical nut on the
other.

Maybe the problem is that the seat slips on the
rod because of large tolerances, rather than the
rod slipping around because the bolts don't hold
it tight.