Pressure relief valve

hi. been looking through the topics but can not find my problem. had a leak from the inlet valve so i replaced the unit no problem. then on checking i had a small leak on the release valve when the water got hot. took the cascade vent off and the valve was leaking from the side. just to old i guess. anyway took this out and replaced with new. done both sides as both as old as each other. done many checks and happy with it. but went to put the vent back on the side of the van only to find the valve is to long. don't seemed to be able to find shorter ones. also found info to say they don't make the old ones anymore. so question is how do people get around it. perhaps a spacer of 10mm all the way around or is there a newer type of vent i have to buy to go with it. my Manuel says its a cascade 11. caravan is a 1994/95. need to get sorted got camp sites booked up. thanks to all in advance.