I started with an armrest housing supplied by Unikarl for £15. I got a bracket and leather-covered lid from Bexley Audi, ordered yesterday, collected this afternoon - combined cost £118.31. They didn't have the 13mm hexagonal nuts needed, but I got these from my local hardware store - with washers - for the princely sum of 40p. Hence, total cost, £133.71.
Working without a template, I cut the hole using nothing more that a hand-held rechargeable drill and a hacksaw blade. It was a perfect fit. The whole thing took less than an hour, and I am delighted with the result, all bar one tiny detail. Here are the pics. Can you spot the one flaw?