Hey I just open out my down bag and then tuck it round me. If I have a mat under me I put a sleeping bag liner over the mat so I'm not sleeping straight on it. It means one bag has two uses and no cutting up of expensive down bags and sewing in a cloud of feathers!!! Also I leave about a foot or so zipped up at the bottom and slip the mat into the bag. Keeps it all together.
I bought one from eBay , can't remember the guy's name but he's based in Oswestry you can send him a message and he'll make it to your specifications.
My mate also Orderd one after he saw mine. Not cheap £200+ for a three season.
All the best, Rob