Yep, perforated drainage pipe, took all the sloshing away in my 750l flat tank. Absolutely no issues now.
In the early days, Ionics used perforated drainage pipes in their tanks that didn't have baffles.
I'm not sure how easy it would be to fit them tightly into a flat tank, though.
Piece of cake spruce, mines got two big baffles in it, but was still sloshing badly compared with my upright tank. I just fed in the drainage pipe and pushed it round the baffles, it was pretty straightforward. I got a lot of it in there, so it really stops the sloshing .
I can honestly say, I don’t get any movement in the van at all now, it’s absolutely spot on.