Usually there are two allenheads to tighten them to the top lip....I always used 3m trim tape....stick it to the inside of the outside mount(facing in to the mirror) and that usually kept from slopping around AND kept it from eventually rubbing the paint off and giving you those pretty rust stripes every time it rained.
Beware of buying HD parts at an "offally" licensed place,the rip off factor is staggering and the quality is no better than after market elsewhere.
Here's some with different mounts and a fair starting price.
http://www.thehotrodgirl.com/peepmirrors.html
good luck.