I've done this with the help of Pelico.
Seat base cost around £20 from Oxted.
Put the seat base in a survival bag, poored in the expanding foam and then sat on it and twenty minutes latter I had an 🙆🏻 shaped seat. Covered it in gaffa tape and it looked quite good.
I positioned myself foward in the seat so my head was lower, and knees up in the dash, but the foam forms around you so you are supported.
Just make sure you have a wee wee before you start 😬
Fellas, smoke me a kipper, I'll be blatting for breakfast.