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I think Gamalot is correct in saying that these Whaler tank mats are best recommended only for metal tanks, either 12 gallon or 6 gallon. The lip on the mats is not very high and only the metal tanks have the bottom configuaration to sit sqarely on the floor to be held in place by these mats
These mats keep the metal tanks in place with no restraint needed other than a little bungee tie down at the back side. Whaler put a strap eye in the boat floor for this purpose.

The red plastic Tempo (now Moeller is using the same mold) 12 gallon tanks fit size-wise, but have rounded and 45 degree mitered corners that lets them slide off the mats. A method of blocking or other tie down will have to be devised to hold them in place. And once that is done, you'd be bettor off with the dri-decking under the tanks.
Same for the large single tanks, unless they have very sharp bottom edges that the tank mats can hold.