My ans: Cheaply.
I bought a pair of Mambo (MWS own make) at Donnington last year for £120. They do not have any cut outs, but if I find I need these when the car is built, I have a mate who can modify them.
I'll try & post a pic sometime.
I would have loved to buy a set of Marlin's carbon seats, but can't stretch the budget at the mo.