I've been distributing universal and custom towers in Europe for about 6 years.
In my opinion, best thing you can do is this:
Get hold of a product called Kitty Hair https://www.sherwinautomotive.com/products/show_product.cfm?product=29744&cat=37
and some thin marine plywood (like 2/8ths thick)
Drill your tower mounting holes.
Cut the plywood to the biggest shape you can to fit along the gunnel of the boat (generally about 12" by 6"). Drill corresponding holes to match your tower feet bolts and ensure that if you offer the wood upto the inside of the deck that these holes match up with the deck holes. Apply about 3/8ths thickness kitty hair to one face of the ply wood (let it begin to cure) and apply to inside of the gunnel in the area you want to mount to.
Again let the whole setup cure more until it becomes fairly solid/rubbery.
Then bolt your tower on, using the backing plates supplied on the inside of the wood.
Tighten the tower bolts until kitty hair begins to ooze out a bit and the whole base becomes fairly firm - then stop!
Go back about an hour later and torque the bolts to the manufacturers recommendations and leave the kitty hair to cure entirely.
In about 24 hours cheque your torques and then use your tower