I managed to get a cap for mine, but I was tempted to drill my original, then plug it after, with a stainless steel nut and bolt and sealing washer.
Hope it helps.
Rustic
as said, MOT here is quite a bit more strict than UK. Also, I have a homologation MOT coming up, to have my T2 officially recognised as utility vehicle. Better not have obvious modifications for that one. There's quite a lot depending on it - yearly road tax reduction from almost 700 to 150 €