The PO removed the fuel tank and didn't keep anything. It's all up to
me now
The original fuel tank mount, which is a 2x4 tabbed to the hull has
some screw holes in it indicating there was a mount at one time. I can
fabricate something for this.

The fresh water mount had rotted away and was removed.
This tank is a rectangular plastic tank, 16 gallons, I think.
The existing mount for this tank, as far as I can tell, was a piece of
plywood tabbed vertical in the settee which prevented the tank from
moving forward. There was a like piece of plywood at the aft end of
the tank to prevent the tank from moving aft, except this piece of
plywood had a 2x4 screwed to the bottom of it for the tank to sit on.
There was no support under the tank except for the 2x4 at one end.

I have fabricated a new frame for the tank to sit on out of 1/2" five
ply laminated together to make it 1" wide, 2" think. When the tank is
installed, it cannot move forward or aft or sideways for that matter.
It can move up and down a little, about 1", so I felt I needed some
straps to keep it from moving up and down. It sits under the strbd
settee and cannot come out unless I have much bigger things to worry
about.
According to Peggy, I might be worrying about nothing.
Am I worrying about nothing?
A few pics of what's going are located here.
Thank you for the replies.
I like the lawn chair strap idea. I also have to re-install the waste
tank. There is nothing left for it to sit on either, but I have to buy
Peggy's book and read it first.

htt ://home.wmis.net/~dhapp/tanksupport.html
( you'll have to make it http )
Here's the rest of the project :-)
htt ://home.wmis.net/~dhapp/page01.jpg