I've found there are two requirements: an angled seat cushion, and some lumbar support. Here's the system I've used elsewhere:
A block running along the front edge rasies the angle of the seat cushion, transfering some weight
off your tailbone onto your thighs. It can either be fixed in pace temporarily (velcro?) or fixed to the underside of the 1/4" ply seat base that is also needed - thicker ply is not required as the flex in 1/4" adds to the comfort.
Stopping the whole thing sliding off the front of the base is kinda useful though - you don't want the little fidget at the exciting denouement of your novel to be accompanied by a sickening 2" drop and sudden stop!
The trapezoidal backrest cushion can be a nightmare to make (though it's the only use I've got for an electric carving knife!) and the foam isn't cheap. You can experiment with a backrest cushion that's a constant 4" deep, providing it's just the right height to suit you.
One problem with tilting the seat base cushion up is that the distance to the floor can become too great for comfort - often the seats are too high without tilting the cushion. This can be vital for a relaxing seating position, so don't ignore it. A couple of 2x2 blocks and a bit of ply can solve this.
Beware that increasing the comfort of a seat often increases the 'length' of the seating position, so that you can end up with your feet pressed into the base of a dinette seat opposite.
PS Sorry about the humanogram's odd hands - he is normally sitting on a motorcycle and wrapping his hands around the handlebars!