[David Hodgson]

When you use production car seats they quite often need modifying as they are bulky around the squab adjuster, this is usually the tightest spot in the cockpit. Look for small car seats that are narrow around the squab pivot.
Like Wazza shows, an off the shelf aftermarket seat without the recline is usually the easiest fit and already upholstered, though depends on the look you're after - modern or classic.
I used a fixed headrest SAAS, similar style to the pic Wazza linked and because it goes right back tot the rear bulkhead, it affords plenty of room for longer legs.