If I'm understanding correctly, you want to be able to frequently remove the ply flooring. I'm not visualizing how you would use threaded inserts to help do that.
How about some kind of hold downs at the seat boxes. Use alum angle with a few fasteners of choice into the vertical part of the alum seat box. However, this might not work depending how the three sections of flooring are arranged.
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I failed to mention the middle seat was removed by the previous owner. I have a casting deck with seat on the front ( which will be rebuilt next).
The boat is 52” wide at the bottom making it 54” on top of the ribs. I didn’t want to just lay the plywood 48” wide so I cut three sheets making the two seems meet on top of ribs. The foam board supports the floor in the gap where the middle seat was. Still my use a piece of aluminum I have (old sign)in that spot for more support.
I also changed plans today after thinking about it. The corrosion I removed because of all the screws the previous owner put in the ribs. Instead of drilling holes in the ribs for aluminum riv nuts I found stainless steel T-nuts and screws , also stainless mending plates I’ll be tying the plywood together with. That and after batteries , fuel tank, (also anchored with T-nuts) and gear the plywood flooring will stay put. Still thinking about using some angle where it meets the casting deck. In the stern where it meets the seats/storage the rib is close enough I don’t need angle.
Now instead of carpeting each section I can do the whole thing in one piece with the snaps. No wood screws and no holes in the boat and I’ll still be able to remove for inspection , cleaning etc.
Behind the rear seat in the aft I’ll be using aluminum angle to secure flooring where eventually I’ll install a removable multiple rod holder for drifting and trolling. That’s still in its planning stage. Right now I have 4 Drift masters on each side.
I’m not mounting any accessories back with any screws in the aluminum.
I’m using industrial Velcro for the rod carrier on the gunnel. I had everything mounted with stainless steel screws previously. Now that I have a little knowledge under my belt I’m not having anything to cause corrosion.
Thanks for the reply and mentioning the angle it’s greatly appreciated.