You can install them on any railings you have. I screwed 10 pieces of PVC pipe on a piece of 5/4 and used copper pipe straps and wood screws to attach to the railing of my friends 30 foot cruiser. We removed them when not fishing.
You can install them on the transom inside the boat or more likely the transom bulkhead. I installed the triple pole holders on each side of the transom bulkhead.
There are plastic adjustable pole holders that mount on top of the gunwale.
Be creative..