Hi,
One problem with making from ply is you cant apply the area rule (unless you're very clever?) essentially the duct tapers from 5"(fan diameter) to around 4-4 1/4" diameter at the rear, the best way I can explain the area rule is the shape changes from circular to a flattened pear shape and back to round, this takes into account the pipe, the pipe is supported at the top of the pear and this allows the cross sectional area through which the air flows to remain a constant taper or nearly so, this really does improve the thrust. You may also need to allow for clearance above the engine head just aft of the fan unit.

When setting up the duct on a d/f model you need to have the duct 1-2deg positive ie high at the tail end with respect to the model centre line or you get rotation problems, I suspect given you want the boat to get on the plane(right term?) then you will need to do the same, suggest you design in some allowance for adjustment of the thrust angle.

The easiest way to make your own tailpipe is make a male plug with a smooth surface. Gel coat it, then wrap with cloth, it doesnt need a great deal of strength so 200gm is sufficient. When cured cut along the top middle, remove, tack with cyano then 1" bandage. This obviously depends on your skills with glass but its not difficult. You can use anything to hand, white foam, blue foam, balsa, whatever to make the plug, doesnt really matter as long as you get the shape and achieve a smooth finish(necessary for good airflow). Mike