If I were you I would get all the little parts from Audi, unless you are really good at fabricating I dont think its worth trying to make these.
Here is the picture I was trying to find (its from
www.vagcat.com, but that site has been down for days). Here you can see all the parts you already have (for the auto pedals) and the addittional parts you will need.
Attached also is the parts number list as a Word document.
As you suggest you can probably use a different VAG clutch master cylinder, perhaps from a Audi A4, I think they all have 30mm travel. You may need to cut a piece of aluminium sheet to cover the bigger opening for the A8 master cylinder though.
Here are the parts I bought from the diagram above to give you an idea of pricing.
Part no-----------description---------------------------------------price £
4D1721465M------metal hose from master cylinder to slave cylinder---71.50
857721453--------rubber hose supply to master cylinder-------------10.96
4D1721485--------slave cylinder to gearbox bracket-----------------11.24
I bought a slave cylinder also but cant find the price. I think it was around £100.