I've just attached it to my rudder, but most people just glue it using sicaflex or sellys allclear to the inside of their yak. Just pick a spot where it isn't going to get bumped. I went without for about 2 years, but now I have one I wouldn't go back. So handy for finding structure and depths. I tend to troll a bit, so they're handy for getting the lure depth right.
Know what you mean about travelling light. I started out that way, then started to carry more and more junk. But now I have the sounder and rod holders sorted out, I'm now reducing the amount of crap I take, and it's so much easier. This is what I've done for the sounder/rod holder mount.
This is the light I've used. Pretty basic, and easy to install. Just attached to the deck behind me. The pole can be removed which is handy, and it comes with a little rubber cap to cover the hole. Also thinking of upgrading the bulb to this led version. Much brighter and a lower current draw.
Both from Whitworths, but probably available elsewhere.