If the Gnomon is made of solid metal then you may be able to get it drilled and tapped with a screw thread and fix it on with screws from below - security headed Torx screws might be better than regular screws.
You may also consider drilling the base and the Gnomon in the manner of a dentist fitting a crown - epoxy in a couple of posts and glue the gnomon onto them.
You may dream of having a one piece casting made of Gnomon and clock face together - or better still buy one ready made from Amazon.
Put BCMW Sundials in the Amazon search box
Good luck...
Peccavi
July 2013
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gardening.