I used the same system but instead of using wood blocs, I used a stack of thin plywood disks. I figured it would best distribute the load, if the T-nut support was round. I used 4 disks of 1/8" ply. The diametre was about 3" for the first one, each of the 3 others are 1/2" smaller than the one under it. This means you start with a 3" disk next to the hull, the one that goes on top of this one is 2.5", then the 2" and finaly the 1.5". You want to finish with a disk that is wide enough to support the rail across its whole width.

For you "lighter is better" freeks, you can drill holes in the bottom disks to lighten up the system a bit.

Bigbob