Bipod for Huntsman Regal XL

Im not sure if this specific item will work for your rifle in particular, however utah airguns is selling saber tactical tube clamps that act as a picatinny rail for air tanks. 

https://utahairguns.com/saber-tactical-bottle-clamp-rail-adapter/

You can give them a call and ask. Something like this would probably be your best bet if you dont want to touch the stock.