The idea of the baffles is to prevent the water all rushing to one side of the tank when you corner/brake/accellerate.
If you have 300l in the tank, and you go round a corner a bit quick, there's a force of 300kg piling up against the side of the tank which could topple the van over, or pull the tank off the floor.
To slow it down you need to cut lengths of guttering downpipe that you can fit through the filler hole, as near as possible the same length as the internal dimension of the tank. You also need to PACK IN ENOUGH TO FIT TIGHTLY IN THE TANK WITHOUT MOVING or else the whole point is wasted as the water and pipes will slosh around as one unit.
Fixing an IBC: you will need to brace over the top of the tank, as the cage is to retain the shape of the plastic tank, not restrain it - it isn't strong enough for that. You should use heavy guage steel strips shaped to fit, or very heavy ratchet straps and the anchor points need to be thick high tensile bolts fastened through chassis members (not just the floor, as this will rip up like tissue paper if you have a crash)
Cheers,
Ian