Here is a quick way to make a control lock. I used some of the Delrin type plastic I had laying around, but anything would work. The pin locates the block into a hole in the front edge of the seat pan.

Then a special aviation-type bungee holds the stick back into the notch.

This takes care of ailerons and elevator. I always straighten out the tailwheel, so the rudder is locked in, and that has worked so far.