Why not spend money on another built in tank?
I have strong aversion against lashing jerry's to stanchions or keep them in the cabin.
On the stanchions it's weight is undesirable while in the cabin it is a potential hazard and a stink bomb.
The smell of petrol is nauseating to the max while diesel is bearable, for me.
Petrol for the inflatable is kept in the vented gas locker.
Diesel, if the 75 liter in the s/s tank is not enough, is lashed down in the cockpit keeping the weight down. (don't start on the color of the jerry's!)