Hi friend, I'm not sure whether you have succeeded in your project yet, but i'd suggest that you design your own joystick as well as its controller. Firstly, you can use the pulsed with modulation way of controlling each of the 2 dc motors, and so your control lines comes to the joystick. I'd suggest you study the ne555 controllers that are reliable to drive high power just similar to the one i've used in my project here.
To make your joystick weather friendly, you should couple the controls variable resistors (if you use drivers that have variable resistors for control) magnetically to your joy controller and put it in a way that you are able to interact more with the controller resistor you lean towards.
Let me know if this helps.