Then baffle it - very simple
I'll put it to you this way, and all my vans have IBC tanks, all bolted through the floor, they are designed for transporting liquids (unlike wyvale tanks) and fully baffled you get NO slosh NO roll from these, unlike wyvale tanks which used to rock the vans something chronic at either 3/4 and 1/4 full.
If anything, the thinner plastic is a better safety feature, as a ruptured tank will disperse the waters energy in all directions quickly and not travel like a ten tonne brick into your cab.
Hope the above helps you understand why an IBC tank is a better alternative to wyvale water butt tanks
Darran