Greetings Doug! My roomate Nick(quad C4/5) and I have been going through the same thing as you over the last couple of years.
Building a lap-tray for the transmitter took some trial and error, as you know geometry is critical to be able to eliminate cross control. The springs for the right gimble had to be modified so they were three or four times as strong as stock, as he has to move his whole arm fore and aft, side to side for control. We were using a buddy box so I could take off and land for him. I had thought of the sip-n-puff idea but it was beyond me to figure how to make one work, also more $$ than he wanted to spend in case it didn't work. When I was unloading the plane(2 meter gas motorglider) the other day, I had the answer when I looked into the fusalage!! The tiny cable pushrod(goldenrod?) for the throttle control!! I made two little L brackets and glued one above the throttle stick on the front of the transmitter and one opposite(below) of it. It was a simple matter to take a 1/4"x1/2" piece of sheet metal and drill a hole in the middle and at each end, unscrew the knob on the throttle stick, slip sheetmetal piece on using middle hole and screw the knob back on. A small spring hooked to the lower L bracket and the hole in the sheetmetal piece works as a throttle return. A bolt on ball and socket goes in the upper hole on the throttle stick. The upper L bracket has a hole sized so only the cable can go throgh and not the sheath.
Then it's just finding the right cable length to reach your mouth.
The mouth control is a clothespin!! hooked up similarly to the other end. Nick bites down to punch the throttle!
Oh by the way, Nick soloed a couple days ago! The grin seems to be permanent!


Best regards, Dale