I priced the Dometic / Ebersparcher diesel tank cap at $30 and was not prepared to pay that price for a cap that cost less than 10 cents to produce. I found two items at Bunnings that do the job perfectly and the cap is much better than the original cap.
You need to buy a 40mm DWV PVC male fitting adaptor from the plumbing department. (see photos below) It has a thread on one end and is smooth on the other end. The smooth end fits perfectly into the inside of the fuel tank filler. About 3 metres from the adaptor in the same aisle is a one and a half inch black plastic cap with a Philmac brand on it, it has a hex shaped base to make it easy to unscrew the cap.
You need to put a little plumbing primer and the blue plumbing glue on to the white fitting before slipping it into the fuel tank filler. Clean the inner of the fuel tank filler thoroughly to remove any diesel and rough it up slightly to allow the glue to grip the inner side of the filler tube. Any fuel resistant sealant or glue will do if you don't have plumbing primer an plumbing glue.
Drill a tiny hole through the white plastic plumbing fitting at the base of the thread before gluing it in place so the tank can vent and does not form a vacuum whilst fuel is drawn from it
. Glue the white fitting with the vent hole down to keep moisture out of the tank. The vent hole is probably not necessary as the cap does not have a sealing ring and will probably let a small amount of air past the thread but I can't guarantee it.
Total price for both fittings is under $6.50!
If you lose the cap again a replacement costs under $3.00