Step 1. Cut the seat board and the foam
The seat board can be cut from MDF, plywood or any other similar type of 20mm (3/4") thick sheet board.
Cut the seat board to fit neatly between the rails of the hall bench, making allowances for the thickness of the padding materials.
In this case, the bench seat opening is 325mm (13") wide by 1138mm (45 1/2")long.
The seat board will need to be cut slightly smaller than the opening to allow for the thickness of the padding which will cover the edges of the seat board.
In this case we allow 3mm (1/8") all the way around for the padding material, therefore we cut the seat board 319mm (12 3/4") wide x 1132mm (45 1/4") long.
Depending on the thickness of padding materials that you use, further allowances may be needed.
Cut a piece of foam the same size as the seat board.