Is their a reason why you are not wanting to mount it within the hull.  The closed cell foam puck is an easy workaround and typically doesn't interfere with the signal

I have a Raymarine dragonfly 4 mounted on the Scotty arm and have never had any issues.

I want to mount it outside due to the sidescan feature - have you found that it works well even in-hull? 

Mojo - any pics of the sink drain mounting? Having trouble picturing this...

Sidescan is definitely a no-go in hull. I use the RAM arm on my inflatable as well and have been considering switching as the arm itself will start to swing up when heading upstream in current. There's always going to be some shakiness with transducer arms; that's just the nature of a long skinny arm with something somewhat bulky on the end of it. 
I haven't used the YakAttack SwitchBlade yet, but the engineering they've been doing on a lot of their recent stuff has been super impressive and I've been happy with their new rod holders and FF mounts. Scotty's ducer arm has impressive reviews on a lot of different sites (better on average than YakAttack and RAM). Just to make your life harder, you might check out Railblaza's Transducer Mount XL as well.
All things considered, if I were picking another one for myself at this point, I'd be choosing between Scotty & Railblaza. Scotty looks a little shorter, so it may not clear the bottom of your hull if you have a kayak with a lot of freeboard; Raiblaza is longer so you'd definitely be clear, but more length means more wiggling. Just my twenty cents.

Do ya one better look here 

http://www.northwestkayakanglers.com/index.php?topic=19707.msg209886#msg209886