You can make your own baffles for the Wydale tanks.
as ive had my 500L upright tank fitted long ways in the van there is no sloshing about unless i brake very hard.much less than the layflat 350 i used to have in the back of my previous van!
tank,frame and fitting.total cost £600.my local garage fitted frame with 6 spreader plates underneath the van.they had to take the fuel tank and exhaust system off to fit it.its solid.