I purchased a 26 ft cruiser this past spring and had to outfit it for fishing. The gunwhale edges weren't wide enough to mount downriggers and the rails are oval teak, so I had very few options. Plus, the one mount I found that might have worked was very expensive. So, I did the board thing, in as much as a buddy of mine had a boat that had been previously set up similarly.

I took a 2X10 that is approx 9' long, notched the underside where it sits on the rails, and finished the entire thing with a quality marine finish. Under the board, I installed eye bolts and use turnbuckles to secure each end down to the gunwhale cleats closest to the stern. The board sits just inside the stern railing, so it's well back in the boat. My rigger bases were installed directly over the railing for support which places them nicely in the back corners of the cockpit. It's rock solid and I didn't have to drill a single hole in the boat. It can be removed and the boat doesn't even look like it's used for fishing (which made my wife happy).

The other advantages of the board are that it gives you a very solid base to which you attach your downriggers, no need for backer plates. Since both riggers are attached to the same board, they offset each other as far as pressure while being used. (I am running the 60" Scotty electrics with 12# balls.) It also gives you a nice edge to mount additional rod holders (I can run five rods at a time). Plus, I attached a bait cutting board and two small plastic trays for holding pliers, files, clippers, scents, etc.

Just another idea, one that makes for a fun project and allows for a lot of options.