Short of cutting the deck out, there is no easy way! If you are seriously thinking along those lines would you not be quicker & easier (& probaly cheaper) just trading it in for one with an underdeck tank?
No idea what consoles & seats you have in there, but I usually carry 2 hulk 30L tanks that fit nicely between the A- frame braces (I guess you might manage 3 across the stern of a Destroyer if you have a transom mounted frame?), and a 22l that tucks nicely under the console - grand total of 82L. I can if need be carry another Hulk 30L wedged in between the two stern tanks & the back of the jockey seat, and also another 30L up at the bow.
It's a pain in the @rse if you want to move about the boat, but it is 134 L, which is enough for most escapades with a 60.......