I've tried to buy one of these via local main dealer, but they are obsolete. Duckworth showed one on their website, but they can't get hold of one from within the dealership network either.

Does anyone have one available?

I did look at the difference between the standard one and this, but the mounting points are all different.

Or can anyone suggest an aftermarket version that will fit?

TransK9 make crates made to measure, in fact I’ve got one myself but make sure you choose the executive seat version. 2021 SVautobiography Dynamic Black ( Black wheels need to go )
1997 C140 Mercedes Benz CL500

I've had one of their cages in the past and they are very good, but this is mostly to stop luggage entering the passenger compartment in an accident - current dog is no longer able to jump up into the boot and has to be taken on the back seat of a smaller car these days.

Not sure if you are willing to import but these seem to have stock.

Land Rover Cary Parts
JLR.CARY.PARTS@LEITHCARS.COM 2021 SVautobiography Dynamic Black ( Black wheels need to go )
1997 C140 Mercedes Benz CL500

Ah, I did see them pop up in my google search. I assumed that they would need to order them in like the other suppliers who still have them showing on their systems, but might be useful as a last resort. Shipping + VAT + duty+ fees might be upsetting though.

I have found a secondhand one on ebay, but missing some of the brackets. It might be easier to get that and try to make up some brackets myself.