Have built up a pair of seats for spare or to sell from the 'good' parts I have.
I figured a way to get the beaded vinyl covers on to the non bead style bases.
The rear flap is held in place by a flat bar, this first has to be removed by drilling out the rivets.
Next the spring clips can be pushed off and the old cover removed.
The foam can be replaced now by gluing to the the frame as per normal.
To fit new or second hand covers of the 'beaded' style the bead first has to be removed.
This is done by cutting the stitching and removing the string. This gives an extra length to the skirt which allow it to be pulled over the frame and clipped into place (first gluing the middle of cover to foam as per normal)
Once cover is fitted the rear flap can be pulled over pegs and held in place by the flat bar using new pop rivets.