You can do this relatively easily on Steel - but it does involve cutting and welding. Can't avoid that I'm afraid - unless you go transom mount.
If you can cut and weld (or get it done), then:
Get a suitable diameter piece of steel pipe (probably ~4" dia or so for most round 1.5" dia transducer heads - you need to leave room for the nut flange on the inside and weld on the outside). Depending on the curvature of the hull you might only need 4" of it, or you might need 10"... Pick compatible steel to your hull steel... you need to know what this is first. Make sure your welder knows what it is so that the right welding procedures and consumables can be used.
Weld (properly - this is going to be your hull skin now) a cap on one end, and drill the 1.5" hole for the transducer, then test fit it. The cap thickness should be the same or more than your hull plate thickness. 1/4" min.
Offer the pipe up to the hull where you want the penetration to be, holding the pipe vertical to the axis of the boat (parallel to the line of your mast). A plumb bob helps here. blocking the boat level is a good idea too!
Scribe/transfer the round shape of the pipe OD up onto the hull (it will be a weird distorted ellipse due to hull curvature). This is the hole you need to cut in the hull plate. Once it's cut, cleaned up and ready for welds (grind back paint
and coatings form the edges both inside and out), slide the pipe up inside the hole and align vertically again. hold in place with magnets or blocking while you level/line it up.
Scribe the shape of the hull on the pipe inside - leave 1/2" or so sticking up past the plate. Remove the pipe, cut it to shape on this scribed line, then re-insert and align. Alternately you can leave it long and trim it with a zip disk later too. Presuming you can access it well.
Fillet weld inside and out, inspect for weld integrity (no holes, no cold spots, no cracks, no undercuts etc etc), blast/brush/clean, and then paint
. If you want to get really fancy, fill rough surface of the welds with a nice thick epoxy
fillet paste before painting and blend it with hand sanding
to make it look super smooth. THEN paint.
Then fit your transducer in it's newly minted housing, sika/3m the crap out of it and you're good to go.