How to waterproof transducer kayak hole?

My Humminbird cable has a 1/4"-1/2" or so head on it...and most of the "wire seals" I've seen are for literally shoving wires thru.

I don't believe I want to cut/re-splice the transducer cable...so how to waterproof the trx cable as it comes out of hole in deck of yak?

Should I be using the "rubber stopper" approach? Can somebody give me a part #?

I ended up cutting a pretty big hole just big enough for the transducer plug to go through.  I put packing tape on the bottom to keep the goop from driipping and them proceeded to fill the hole with goop.  After it dried for 3 days I tried tilting the head up and down and it pulled the cable loose from the goop so there was an area around the cable where water could run through.  I ended up gooping over this area over and over again until the goop was so thick around there nothing gets through not even around the cables.  There is probably a better way to do it but mine doesn't leak yet. I followed these instructions when I installed mine and it worked well. http://kayaksportfishing.com/equipment/submissions/ted/fishfinder/fishfinder.htm

I was thinking maybe a cylinder of closed cell foam with some marine goop could work too.

I think I need to do a launch BEFORE drilling holes and installing sonar...just to get a feel for the launch.

Done plenty of bodysurfing/spearfishing in ocean and am familiar with getting past breakers...but just not on a kayak.

For a Humminbird unit the I use a Method similar to Travis, but wrap the cabble till theur almost a big or bigger than the hole, shove it through, andthen goop the heck out of it.  For other units where the cables exit the back a hole cove like the one that comes with Humminbird units is great.