Main issue I would be thinking of is 75 kg in the boot if you haveca serious accident, or even emergency stop.
Maybe pure freedom trolley, then tuck 2 x 25l barrells in rear footwells, then two in passenger footwells
provided layed flat the ones behind you should be reasonably safe if you brake hard.
But again, even trolleys become pain on rear, better off if serious getting a tank bolted proffessionally in a van.
Mr B