This is a square section rubber,measure the clearance from the bottom of the slot on the air box to the bonnet add approx 8mm for compression and measure the length and go to a supplier like Grippy Rubber Products who should be able to assist. This is if you can't get one from Mick's Motors,Tooley Imports,Vintage Veedub etc.
Thanks. I went to Clark rubber and found something very similar. I glued the two ends together and put in. I also managed to get some sponge rubber discs and fixed the fresh air dampers