Sort of thing you might get from Foulkes Chandlery Barge at Bursledon, Hants. Yes, I know it's a long way from you but it wouldn't be difficult for them, or some friendly forumite, to post one to you.
If the grid you have as a flat grid plate on the top you have a compass based on the sestrel 4" lifeboat compass it was called a "medina" any gimbal system off of 4" sestrel will fit not much help I know but the part number you need is 5901 gimbal assy
Regards
bpsc-marine.co.uk
Hi I also had this problem,
if it is as I suspect the wedge type you need I would make a wedge shaped holder using tufnol (look on e-bay for small pieces) I would use either carp or whale brand as these have good shear resistance you can cut it with a hack saw and use a normal carpenters plane to smooth the edges then attach to your boat with countersunk bolts or screws.
hope this helps
regards
john