I just realised kartbay is in Oz...
Are you in the USA?
If so, this seat might fit ya:
http://www.seventh-gear.com/store/proddetail.php?prod=14798
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I've made race seats wider by placing a hydraulic jack and wood 4"x 4"s in-between the seat sides,
and then heating 1 whole side with a couple large hair dryers or an Industrial heat gun.
After I got 1 whole side
really hot, I slowly/gradually spread the sides further and further outward with the jack while continuing to heat with the hair dryers.
Then I simply left the jack in place until that side completely cooled so it would retain the new wider shape.
Then I simply did the same thing to the the other side of the seat.
I've successfully made my race seat sides spread a couple inches further apart this way so they dont squeeze ya so tight.
(i'm a pretty big dude too)
You may have to get inventive and devise some method to keep the jack and 4x4's from popping out of the seat while spreading.
(like tying the jack to the seat with a rope and using C-clamps to hold the 4x4's in place against the sides ...
I didn't invent this method, Its just a little trick that kart racers
(that gained weight) have been doing for quite awhile.
You'll want your race seat to fit very snugly to hold you in. It's normal for the sides to feel just a little uncomfortably tight when you first sit down, but then after riding a short while your body will conform a bit and you'll be glad the seats a little on the snug side to help hold you in around those fast corners...