Why not get some top hat fittings made like the ones I had done for mounting my rear bench seat.
I got 6 x 3" dia disks of 3mm Stainless then had a 30mm long piece of 20mm dia. stainless roundbar welded onto the center of each disk.
Then each disk had a hole drilled and tapped for a 10mm SS bolt going through the center of each disk and into the roundbar to a depth of about 20mm leaving the roundbar sealed off at the end.
Then each disk has 4 x countersunk holes drilled around the disk that are sized to take no12 CSK SS self tappers.
Then you just drill a 25mm hole in the deck to take teh roundbar and the 4 holes for the self tappers and screw the whole things down with lots of Sika.
Very strong and worked a treat !