here is how I did mine.
1) Cut two pieces of 1/2" plywood to the dimensions of the top of your bench.
2) Cut a piece of steel/aluminum plate to match the dimensions of an 8x8 swivel and drill holes in the corners to match those in the swivel
3) In one piece of the wood, cut a square hole that is slightly larger than the piece of square plate you just cut. (this hole should be more towrds the front of the wood to allow room for steering a transom mount trolling motor if you have one)
4) Glue the two pieces of wood together with contact cement and put a few scres in them to hold them together even better grind off any tips of screws poking up.
5) Coat the wood in two coats of fiberglass resin
6) take the steel/aluminum plate an place it in the newly formed "pocket" from where you cut that square hole and put the two pieces together. place it in the hole and drill holes where the holes are in the steel plate
7) carpet the wood
8) make a small platform a little bit bigger than your swivel because you will need a platform for the seat and swivel to sit on or the back of your seat will scrup the top of your bench
9) use the plate as a template for the screw holes here as well.
10) place the seat and swivel on top of the carpeted platform you just made and then place them on top of your carpeted bench top. make sure all your bolt holes line up. place a stainless/galvanized carriage bolt in the holes. on the back of the wood where the pocket is, place you steel/aluminum plate ontop the bolts and put nuts on the bolts and tighten down.
11) place new assembly on the bench and predrill some holes for some #14 Stainless steel sheetmetal screws that can be bought at your local Lowes Home improvement. they come in a bag of 5. place a screw in each corner and one either in front or behind the seat---DONE
^^bottom of wood where pocket is. the steel/aluminum plate goes here. this pocket's purpose os to allow clearance for the plate and nuts to fit flush under the wood and allow the wood to sit flush on the top of the bench.
^^^this pic is missing my platform. I found this out after putting it together. I had to make a smal carpeted platform to raise the seat up more so it wouldnt scrub