Fuel tanks
 

Not sure if any of the larger SeaCraft could use these or not. They are not square or rectangular, but may be able to be modified.
I have two ~80 Gallon aluminum "saddle tanks" originally fabbed for a 29' Phoenix by Sunshine Welding in Port Canaveral. These tanks are not "new" but unused. They have new sender units, two bosses per tank (can be used for gasoline or diesel) and always stored inside. These were built to sit along the gunwales of the boat so there is a beveled side that can be seen in the pictures.
Sold as a pair. Dimensions 68" L, 13 3/4" H, 20" bottom width, 25" top width.
Price is $700/pair OBO.