What's the best pedestal to seat bottom fastener?

[fouroaksphoto]

I'm working on a Springfield bench seat installation and was wondering what is the best way to fasten a seat spider to the seat bottom substrate which is plastic 3/4" thick. The folks at Springfield havn't gotten back with me but a tech at Wise Seats said they use T-Nuts mounted on the top side of the seat bottom. I can't get into the cushion to use T-Nuts but I'm considering 1/4"x20 threaded inserts. Anybody used inserts before?