and here's the GG&G cantilever ring mount - it'll set the aimpoint at the right height to co-witness (lower third of the tube).
http://gggaz.com/products/apcantring.php
and here's a riskier but cheaper place to get one (make sure you call to see if they have it in stock before ordering):
http://www.botachtactical.com/gggaimcanmou.html
cheers,
MM