i've seen somewhere a plug for the return line (can't remember where) and it was basically installing in place of the FPR valve - if everything else is the same you could look at that. I did my own with an old FPR valve - i threaded the inside of the nipple (I think i used a 6mm tap), i put a screw in with teflon paste and then grind down the hex head to the same diameter as the outside of the nipple - took about 1/2 an hour but had no leaks. To make sure you could also add a short piece of fuel hose with a plug at the return and clamps, that way you're 100% that there's nothing coming out - just in case everything else is the same this could work - i can take a pic of it if you like.
