This was on a '98 Malibu VLX, but should be similiar for just about any boat. I also made an effort to use just basic tools so it could be reproduced.--
Allright, I started with a locking swivel from West marine ($20) & two sheets of .25" plate aluminum 10.5" x 13" ($20) from a local aluminum fab. shop. I used stainless steel fasteners so pretty much everything is corrosion resistant.-
I marked & drilled holes for the bottom plate using the seat as a template. I used the bottom as a template for the top plate as they both would be the same. I first tried the swivel just back from center but didn't have enough clearance so I moved it all the way to the rear of the plates-
I was concerned about stability in the forward position so I added a roller to the front of the assembly between the plates. I had this already but I'm sure you can pick them up at a local supply warehouse.-
The total height added is around 1.5" (Just what my VLX needed)-
I did some minor touch up work that I didn't take pictures of but this should give you an idea of the basics. I also had to notch the seat base above each fasteners so it would sit flush. In the forward driving position it is rock solid. It does have a bit of vertical movement when swiveled to the side but I can live with that. I am keeping an eye open for a better swivel possible from a different industry as it is the weak point.
Much better-(no the speaker belong there)