1) Choose if your going internal pump or external.

2) Dont make each tank flat bottomed but make it with a sump area at the lowest possible point.

3) Pick up from this sump area if internal or take feeds from it if external.

4) No need for baffles with the above, and foam if fitted, just stops it sloshing about.

5) Ebay items are overpriced - seen those for less than £75 elsewhere.