We have a Rumba that I bought for my daughter (named "Rumb-baba") and have the rudder in our garage. The rudder blade is made from solid spruce (plywood will not work). I will be happy to temp-plate it for you if you like. The trick will be to come up with the rudder-head fitting with the retractable pins to allow you to mount/dismount the rudder. Doug Fisher of Ullman Sails in Sarasota, Fla (941) 951-0189 used to sell the boats and may have a source for replacement parts. Let me know if you're interested.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."