Make/buy a cargo net that goes from the beam across the rear of the front seats and is anchored/clipped to the seat belt upper mounts on either side.
Do you have Helipods? Do a google search. Made in NZ. Just the cats pyjama's for the R44. Makes it into a useful helicopter.
Barring that, just toss the clubs in using the seatbelts. Its only going to be an issue if you crash, or you go negative and your balls come out.......
