I’ve got a plastimo
water tank under the q-berth and a vetus holding tank
under the v-berth. Going on ten years now.
Didn’t really see any need for “underpayment” as there were no rough edges there. The Plastimo came in a stout nylon jacket. Tied off the corner loops to D-rings screwed into the underside of the deck
Note that when empty/not used, the tanks take up zero volume and other cruising gear
can be stashed on top of them. When full, they take on cylindrical shape and may push-open the lids of the compartment.