on my last boat, the fuel tank was square and (as I found out during a gale in the Irish Sea!) the fuel sloshed from side to side and uncovered the drain to the fuel filters etc so lots of air….
baffles were an idea but meant cutting open the stainless steel fuel tank as a well as the fibreglass housing.
simplest solution was to carry two x 20 litre cans of diesel something I now do on my 38 Classic. that way always half full. simpler solution than a day tank