An easy way to add rod holders can be done without actually adding any more rod holders or making other mods to your boat.
I use a Penn downrigger base that just slips into a standard rod holder. Then I added a Bee Ready rod holder setup onto that.
They come in various sizes for multiple rods. I can troll up to 8 rods out the back of my boat by using just 2 of the standard factory rod holders with 2 Penn setups. The Penn downrigger base comes in 2 sizes to fit several size rod holders so be aware when buying one.
It would also be an easy project to make one if necessary to fit any rod holder in order to support the Bee Ready.