upgrade seats

[chaddemler1980]

my boat has no seats and trying to find some originals is getting hard, i decided to buy a captains chair and mount it myself with the original mounts. i have also built a "wrap around" seat pretty much to what was originally there except I left an opening "did not stretch all the way along the rear" to allow easier movement to and from the swim platform. but i wish to add 1 or 2 "swivel" seats to the boat, can i do this without damaging the boat? and how do i reinforce the underside in the area i wish to "add" the seats? i do not want to add a plate on the top but rather it be hidden under in the engine compartment... but there is no support in the areas i wish to add the seat and many of my frineds are 280 pounds plus in weight so i dont want a big chunk of my fiberglass floor being ripped out. not to mention i do not know anything about adding fiberglass to a boat... so will a large plate "such as alluminum work say 1/4 thick and atleast 12" x 12" in size" or do i need to go bigger? and what is the best way to secure this plate to the floor or should i just rely on the pressure of the seat mount bolts to hold it tight enough agains the fiberglass floor "pinch". i should add the seats i wish to buy do come with an aluminum plate roughly 6" x 6" already attached to the bottom, i am asking if adding another plate to underside will do the job? kinda of sandwiching the fiberglass between the two plates, or will it just rip out anyway? i can add a bigger plate underneath i just dont know if the fiberglass is strong enough to support a big guy in a single post seat without cracking or pulling through or dropping through for that matter. the areas where the seats will be added are strong but there is minimal support "mainly just for walking" such as the motor cover.