Here is a inline mountable Scotty Rod Holder Mount to be mounted in rail. It is much more minimal then the larger '4 screw, non inline version' but it does not compromise on strength.
It measures 58.755 x 116 x 56mm (H x L x W)
There are also three spacer sizes that are to be used if the rail is slightly recessed into the hull. They stop the base of the mount flexing upon tightening and allow the nut to be tightened hard (Without causing too much stress on the base) by bringing the rail height up flush with the hull.
The spacers come in three sizes; 2mm, 1mm, and 0.8mm. Use what combination you need to make up the recess.
You will need two M6 x 30mm sliding rail bolts with wing nuts (See picture). They did need to be filed about 1mm to fit into the track on my Wilderness Systems yak.
I purchased these ones here.
Support only needed under internal locking tab