Mounting seat pedestal deck plates

Mounting 2 pedestal deck plates to my boat for side by side spider rig fishing but I can't get underneath to put nuts on the bolts that will secure the plates. Do y'all know of any anchors or bolts that I can use to secure them. Maybe some kind of expanding bolt. Don't have the money for a double seat. 7 month old took care of that! I found some toggle bolts that are used for mounting trolling motors they are expensive. $4 a piece and I need 12. May need to go to fasten all. Figure one of you guys may know of something. Thanks!

Don't know the cost and can't remember the name, but saw some posted that guys were using to mount trolling motors. Sold at home depot or Lowes. They have a plastic piece that holds it together when you insert it into the hole then it pops off when bolt is tightened. Guys had used them really liked them. Not your old fashioned toggle bolt. You mounting on Aluminum or Fiberglass?

Aluminum deck.

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