Bite the bullet and install a through hull transducer properly. Lay up a 2" (or whatever the correct clearance hole size is) fibreglass tube with at least 1/4" wall thickness, cut to length and to square up the ends and glass in place through the hull. Build up a nice fillet on the inside and outside, glass taped at the root and on its surface, with a structural paste of milled glass fibres, microfibres and some silica gel to prevent sagging in between, then once that's reached a good 'green' cure, build up a fairing block in solid GRP and finish with gelcoat.
You may want to worm the transducer thread with a piece of stout whipping twine with a loop tied in the top end before applying sealant and bedding the transducer in place so you have a seal-breaker to cut the sealant if you ever need to get the transducer out without damaging the mounting tube.
You probably wont get good results from a levelling block and an in-hull transducer. Varying acoustic impedance across the transducer face will disperse the beam. The best bet is probably glass or sikaflex in a tube cut to fit the hull angle at the bottom and immerse the transducer in it in oil or low melting point grease such as Vaseline. There must be ABSOLUTELY NO AIR BUBBLES, so if you use grease, melt it with a hot air gun! The transducer face must NOT directly touch the hull. To find a good location, put the transducer in a plastic bag full of water with a rubber band round the transducer thread, wet the hull with a sponge and hold the transducer in the bag against the hull and try the depth sounder. Keep vertical!
The silicone solution is only good for thin hulls with a flat bottom (less than 10 deg from horizontal).