A dabble into upholstery
How to upholster the seat:
Modern upholstering is done (in some cases) a bit differently to the more traditional methods of the past.
This padded drop-in seat consists of a solid seat board with a layer of foam and a layer of batting which are then enveloped by a fabric (such as calico) which is fixed to the underside of the seat board holding everything in place.
The seat is then covered again, by a finishing fabric of choice.

Below are the instructions for making a drop-in padded seat.
The materials list can be seen here and the tools list can be seen here.

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.

Step 2. Trim the edges of the foam
Cut an angle around the perimeter of the foam pad.

Start the cut slightly more than half way down the thickness of the foam and angle it in about 45 degrees.

When the cut is complete, flip the foam pad over.

The cut is to be on the underside, not the top.

Step 3. Glue the foam to the seat board
Glue the foam pad to the seat board.

Spray glue (or apply contact adhesive) onto both surfaces, the seat board and the underside of the foam pad.
When the glue is 'tacky' to touch, press the two surfaces together. Press firmly all over.