I saw a condor parang sheath made from old car dash board, it's basically grey kydex. The guy had done a really good job of it, i'm assuming that a kydex sheath of that size would not be cheap to commission, at least £50. If you decided to make your own aluminium pop rivets and washers would work just fine aswell, you could also paint them black if you want to be fussy. A one off job you could use old kipmat sandwiched between two planks for the forming .