1/4"-20 Size Stainless Steel Toggle Bolt Anchors, 6 pack - Garelick

These work great. My only complaint is they are too pricy for what you get.

I used a six pack of these to add another pedestal base to a boat with a 1/2 inch plywood floor. I had installed other bases before putting the floor in, but these seemed like the best option for installing once the floor was riveted in. One cautionary note: if there is flotation foam all the way up to the underside of the floor, you need to remove it to about an inch below the surface and about an inch in diameter. This will give you space to swivel the toggles flush with the underside of the floor. Once I figured that out, installation was a breeze. I chucked a hex key (L-shaped) with the long leg in a drill, inserted the short leg in the hole, and chewed the foam up below the hole so I could install the toggles. Plenty of polyurethane adhesive (3M 5200 or equivalent) completed the installation. I haven't yet put it to a stress test, but I just can't see the toggles pulling through the plywood.

These anchors worked very well and were easy to use. A well designed product. I don't think the seat I used them on will ever move again. My only complaint is the price. I think the price is too high because it's a "marine" product. But I wasn't able to find another alternative so I guess you have to pay the price.