If you're planning on one of those, why not a car seat? I think I suggested this to you before, right? Anyway, I got a pair of bucket seats from eBay for £100 and have used it in my pit and it's still going strong compared to the last £99 office seat I bought which only lasted ~6 months.
Obviously you will need to design and build a "frame" to which you will mount the seat, but I think this is a big plus as you can then design it to suit your needs. A few bits of lumber, MDF, and paint and you're done! Way less than £300 for everything as well, even if you had to buy tools such as a jigsaw or power drill.