I've been looking at a number of fixed back seats and the majority of manufacturers like Cobra, Corbeau and Sparco etc do a number of base mount or side mount fixed back seats. I'm currently looking at the either a Corbeau or Cobra Monaco S which are both base mounts. The Monaco S costs around £120 plus another £50 odd quid for a Golf base mount.
My advise, go to Demon Tweeks website and place an order for both the Motorsport and Road Tuning Catalogues (both are free with free delivery). You'll find a good selection of seats with dimensions and details of which are base or side mount.