is there any possibility to retrofit a roll bar to the RV-3? Unfortunately our RV doesn't have one and we fly from more or less "good" grass strips only.
There are two drawings for roll bars in the RV-3 plans if I recall - and I have seen many other designs by various other builders. Take a look at the plans and they may give you some ideas. It will be somewhat invasive.
You should be able to cut holes in the top back skin behind the seat back bulkhead (same principle as cutting flap actuator holes) and feed the two steel tubes through them and then weld them together at the top (U-shaped top steel piece) as per SK-54A. You'll probably need a 3/16" angle drill to drill the attach holes in the longerons.