Find the ribs or stringers in the hull if you can and ensure that you make some attachments there with Stainless Steel screws. Build up the area with a box as mentioned, or I like adjustable pedestals that rotate sitting on several layers of built up plywood, marine if you can get it, extended out to the edges of the seat or more; 2-3 layers, stair stepped to the pedestal mounting (dimensions).
I like the increase in room, sitting higher being more comfortable, being able to rotate at least 180* and as plush a seat as you desire...arm rests, lumbar, headrest, on and on. Other thing is that if you use the 9" bolt patterned pedestal the post is usually 2" and on some boats like Crestliner, the seat can be locked in the forward position for steering, or simple depression of the lock button and you can swing around. Then if you want to move the seat (and pedestal) out of the boat...out of the way when not in use, or move it to another location just takes a second to pop it out of the mount.