I'm toying with installing a corbeau safari seat- my old one was destroyed beyond saving. At 2/3 the cost of the re-trim kit, the entire new Corbeau seemed a better deal.
It should be obvious which brackets are the old defender ones. This is a crappy cell phone pic with no measurements- but you can see that what comes with the seat is pretty close to the OEM.
And the width of the seat is very very close (if not exactly the same) to the original. The problem is that the mounts on the stock seat are further forward than the Corbeau(on the seat base itself), so if you try and use the same location (for the bracket in the tub) the seat is too close to the front seats.
My point in all of this- you can get the brackets for any fold and tumble seat, and they will work with the defender seat. And, the defender brackets (at least the front) will work with the Corbeau seat, at least.
If you decide it's necessary, I'll measure it up for you.