I wouldn't do it. I looked into it myself and decided to go with something a lot less expensive, and less "permanant".
Make a holder in the shape of a large capital "H" with PVC pipe, two PVC "T"s, one PVC cap and a thumb screw. The PVC cap goes at the bottom of one of the H's legs. Place the leg of the H without the cap over the seat pedestal post, and then put the seat back in place as usual. Place a large beach umbrella / patio table umbrella (you can get some good ones at pool supply places) in the other leg, bottoming it out into the cap at the bottom so the pole doesn't scratch up your carpet. Drill a hole slightly smaller than the thumb-screw's diameter into the pvc pipe leg that the umbrella is in, and then screw the thumb-screw into that hole with pliers for the first time - making threads for it. You will have to screw it in, back it out, screw it in, back it out, etc... like you are using a tap. Once you get it through and worked in enough to tighten with your fingers, tighten it around the umbrella handle.
Nice thing - when you move, collapse the umbrella and place it on deck. Strap it down with a rod holder strap. The holder can stay in place, as it doesn't bother anything when running your boat.
You can buy the piece I mentioned above at several spots, but I can make them for about a 1/4 of what they charge for them.
Just another option.
All the best,
Glenn