Friend wants suggestions for mounting a side imaging transducer on a Trident

I have a friend who is thinking of buying the Humminbird 698ci HD SI fish finder for his 13' Trident with the XPTH 9 HDSI 180 T plastic through-the-hull transducer.  Because the 698 is a side imaging fish finder, the transducer needs to be mounted in a position where there is nothing on either side of it.  Consequently, it can't be mounted in the Trident's scupper pocket or in a through-the-hull mount.  My friend was thinking of using the leveling block to drop the transducer down below the level of the keel, but I suggested that it was a really bad idea to mount the transducer so it sticks down several inches below the bottom of the kayak.  I suggested that he get a standard transom mounted transducer, and either mount it on a transducer boom, or mount it on the stern of the kayak directly behind the keel, using the cast-in-nuts which are for mounting a rudder.  Have any of you mounted a side-imaging fish finder on a kayak?  Do any of you have any suggestions and recommendations for how my friend can mount a side-imaging transducer on his Trident?

A deployment arm would probably be the best bet. That's what I'm running on my Trident on a DI and happy with it.

I'm using a RAM transducer arm for my Humminbird side-scan transducer.  It works pretty well.  It does introduce a little bit of drag though.

The transducer designed for use on the Trident is the XTH 9 20 P and customers are charged $75 for swapping their new transom style transducer for this one.

Side imaging is not going to work in salt water and not sure where the hi-def function would be a benefit. I have yet to read positive posts about the HD on any finder as to its being useful.

With the 20 P transducer your friend gets the dual beam that he will use and the transducer is designed for the kayak. This transducer works with any of these Humminbird finders:

141C, 161, 323, 325, 343C, 345c, 363, 365i, 383C, 385ci, 515, 525, 535, 550, 560, 565, 570, 575, 580, 581i, 585C, 586C, 587CI, 595C, 596C, 597CI, 717, 718, 727, 728, 737, 747c, 757c, 767, 768, 777c2, 778C, 787c2, 787c2I, 788C, 858c, 931C, 937C, 957c, 958c, 1157c, HDR 610

MATRIX 10, MATRIX 12, MATRIX 15, MATRIX 17, MATRIX 20, MATRIX 25, MATRIX 27, MATRIX 55, MATRIX 65, MATRIX 67, MATRIX 77C, MATRIX 87C, MATRIX 97

PiranhaMax 10, PiranhaMax 15, PiranhaMax 150, PiranhaMax 160, PiranhaMax 170, PiranhaMax 190C, PiranhaMax 20, PiranhaMax 210, PiranhaMax 215, PiranhaMax 220, PiranhaMax 230, PiranhaMax 240, PiranhaMax 30