We began by building a simple frame of 2" x 4" lumber. With the water tank at its final elevation, we know where the holes for the fill and vent should be located. Only the fill is present in this picture.

Woa, blurry!

A diplucate frame is built and supported by 4" x 4" members, 16" tall (the height of the water tank). Three sides of the box will be skinned with 1/2" plywood; the side facing you will be left open for storage.

The top will be covered with 3/4" plywood.

The vent has been added (near the fill) and the outgoing PEX pipe has been added...seen peeking out from under the right side of the tank.

A mock-up of the pieces to the freshwater fill.

(EDIT: The ribbed supply hose was later replaced with braided PVC pressure hose.)

The freshwater fill apparatus, closed and...

...open.

A test of the water system! There are a few leaks, but nothing insurmountable.

Water flowing from a faucet never looked like much...until you're the ones to make it happen. :-)