Call me tight but I'd be tempted to make some (he says with a multitude of half finished projects on the go) out of 1/2" marine ply and some deck eyes with webbing straps.
For holding them down, I'd through bolt, certainly in way of the retaining straps with a backing pad behind to transfer the load.
If you were to put a deck eye port and starboard of each tank as well as fore and aft, you could dispense with the base or replace it with a bit of treadmaster or similar.
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