I've been using rod leashes ever since I started kayak fishing - they have saved a few combos on more than a couple occasions. What's funny is that I haven't rolled over or had some crazy situation that has caused a rod to go for a swim. It seems to always be something stupid like me bumping the reel out of a flush mount rod holder with my paddle or just plain having a second of clumsiness.
For the connection point to the rod, I used to use Velcro's "One Straps" that were cut down to size and zip tied to the coiled leash. This particular Velcro (it comes in packs of 3, about 7/8" wide and maybe 18" long each) holds up well and works even when completely saturated.
I switched over to these ratcheting DIY leashes in the spring of this year and really like this method as it is easier to connect/disconnect the leashes to the rods w/one hand as compared to the velcro. They are also very inexpensive and easy to construct and the cuffs and are quite strong. Some very simple modifications to the ratcheting cuff make it perfect for this application. At around $4 a leash, it is a cheap investment that is well worth the 5 minutes of build time!
Here are a few preview shots and as usual, the full build details are on my Palmetto Kayak Fishing Blog HERE