I had a BW 23 Outrage and we had in each floor fishbox 3 short, fat 12 liter steel tanks and the six with BCD mounted one on the floor, tanks with BCD mounted will not roll around
I saw the clip style tank holders but disliked them, on the dive panga we had bungie cords to hold them in place and on the Venture 34 i could store 16 US Aluminum tanks in the huge locker center boat, mounted tanks on the floor.
Bungie cords (or the clamps) mounted on the inside of the gunnel should work if you find a way to keep tanks bottom in position, the dive panga had rings made of a sturdy grey plastic tube (probably a high pressure water pipe - a bit wider than the tank. Unfortunately i don't think i have any photos of this system
If you want a stable and safe solution either for center cockpit or two on each side then have welded a tank rack from SS tubes with 4 tubes sticking out on the bottom.
This tubes go in 4 a bit bigger tubes -with the bottom welded close to be watertight- to lock the rack in place but lifting the rack up it comes free and you can store it on land if not needed.
If you are interested in that i can make you a design.
Chris