You can use Hobie rudders, have a look at the link below, it will answer all your questions, well to do with a Mossie anyway.
http://home.vicnet.net.au/~nmcca/doc/Mosquito_BRR.pdf

Hobie rudders would be fairly heavy wouldn't they? Mine weigh about 1100gms each. Check the thickness of the blades too, if your using square aluminium frame rudder stocks the blades should be around 19mm thick, hobie rudders look a bit thicker to me though I haven't measured them.
They should steer alright, if they fit.
Darryn
1704