You can try to use an adapter like this
To convert the M68 thread on the OAG to M48. Then you can connect a 2" extension tube on the M48 thread and stick that into the focuser.
Please note that I am not 100% sure if this is the correct adapter. But searching for M68 on TeleskopExpress will show a LOT of adapters and the one you need may be there. Also note that this and similar adapters add distance so you may need to solve the back focus puzzle again.