I think I would be tempted to put a very low stand (4x2 length ways and 20mm ply on top) and sit that on top of a sheet of PVC (or anything very thin and waterproof) then run thick beads of no more nails along the undersides of the 4x2 and then sit that on the PVC.
Side on:
| SUMP |
| |
|________________________|
######################### <- poly
========================= <-ply
+++++++++++++++++++++++++ <- 4x2 stand, only 2 or 4 inchs high
========================= <-no more nails, then pvc underit all
End on:
| SUMP |
| |
|____________|
############# <- poly
============= <-ply
++ ++ ++ <- 4x2 stand, only 2 or 4 inchs high
== == == <-no more nails, then pvc underit all