A Google search for design spec yielded many hits but none were official spec just some DIY tips and photos of "look how I did it" Onbe website 'Tank Tek' gave illustrations of their commercial offerings. They used welded plastic and stipulated spacing was 250 - 350 mm I can only assume the close spacing was due to flexible material. Elsewhere i found mention of baffles must be same thickness as side walls. Some details of welding stipulated both inside and outside fillet welds be used on aluminum. One DIY photo showed the top of baffles below top and not welded. This is a recipe for problems. The unsupported edge will bow under normal sloshing impact. Another showed no cut outs at top thus forming pockets wherein gas can collect. Not to mention which the tank cannot vent properly. We stipulated button hole welding for baffles to top of tank. Because the welder has to continuously weld each seam inside not tack or spot, the top goes on last. A slot is cut along line of baffle so welder can weld down thru button hole and fasten top edge of baffle. This is only place where non continuous beads are used. The exception being where tank is large enough for welder to enter by way of inspection hatches. Dangerous work. A friend was asphixiated doing work like this because he forgot to bring breather mask with air line. If a swing arm type fuel gage is called for take care to position it so arm does not hit baffle. A small sump below fuel pickup is recommended and of course tank bottom should have a slope for water accumulation to drain to sump. Sorry but memory fails me for remainder of details and I seem to have lost the original CAD drawings I made. Its been a few years now. Arild