The clamp has a 2" throat and is rated for "Lake-Troll, Sport-Troll, Easi-Troll and Mini-Mag downriggers".
You could also fabricate you own mount.
Thinking out loud and without drilling into the boat :
Bottom
1/4 or 3/8 Alu flat bar x (width allowable) spanning two of your handrail stand-offs (spacers)
Top
1/4 Alu plate for the pad drilled for downrigger base and sized accordingly
Outside edge
Angle 3/4 x (ht. necessary) to level the pad (cont. welded to bottom of pad to make level)
This piece (3/4 side) would meet the top of the outside edge of your boat. You could make it 1/8" shy and apply 1/8" industrial rubber with contact cement to buffer contact with the top edge of the boat.
You are pinching over the standoffs. If done right you will get a very strong purchase. You can through bolt or drill and tap (flat bar) using something like 5/16"-18 flat head stnls fasteners. The Allen-drive flat head fastener would be a nice choice.
You will want to drill the flat bar while you are square to the finished height of the pad if you are going to tap.
The two main fasteners could be placed under the downrigger base out of sight.
You would need that one weld to keep it sanitary. The rest you can do at home.
If you don't mind drilling into the boat, that might make it a little easier to deal with the leveling riser. Then you could use flat bar and drill and tap (or through-bolt) into the boat's top edge tube.
Use Nylock nuts and be careful not to crush the tube.
Here's a different project but you can see what the main fasteners might look like:
http://www.ifish.net/board/showthread.php?t=299117
Or take it to someone who is good with Alu (marine) fabrication and let them create.
That is a lot of monkeying around if you don't like doing that kind of stuff
Downrigger Mounting
DirectDrive
Linear Mode
