Your foamie that uses tape as the hinge... the hinge is sealed by the tape... so its sealed already[8D]

Tthe 1/2 inch gap between fixed and moveable surface is a problem... could you try to re-hinge it with less of a gap? that wide flexible tape area is going to allow the hinge line to flex... that can reduce the ability of the control system to accurately positin the rudder. take your fnger and gently hold the trailing edge of the rudder as you move the stick and watch the hinge flex while the TE of the rudder sits still! that means... you have inadequate control not due to inadequate throw... but due to hinge flexing.

Can you post a pic of it? we may be able to guide you in the best solution.