I have a little experience with this, I hope I can help.
If the sidewinder has about 3" of travel at the joysticks current length, you will likely quadruple that if you add 2' or more worth of extension. I think your best bet is to build your own box to house the joystick guts, pot, and put in your own spring (which would probably an industrial one to boot).
I'd make sure the extension arm has a nice firm barrier, the amount of force you can apply with that much leverage will be a lot more than you'd normally see with a joystick. I'd even go with steel and cut out the area for the arm, I'd also do that aspect at the end of the build so you know how travel you want in the stick.
I'm curious what Homeboy would day, he is the closest thing we have to a stick expert on these forums.