Any ideas on how to do this? My tank is about 1 1/2" and my filler is 2". I've seen some plastic stepped adaptors but they're not marked in any way and I'm thinking may deform if I put jubilee type clips on them. Has anyone got around this?I'm thinking of a stepped aluminium adaptor or a flared aluminium tube or summat. Just after ideas.

Burton Power do a stepped stainless exhaust component. Follow the link below. The item is half way down the page. Unfortunately I think it might not quite work for you as it is stepped from 38mm to 45mm. The overall length may be a problem too?
https://www.burtonpower.com/product_main.aspx?home.aspx

make one? cant it be done with a 2" tube and a 1 1/2" tube nad then a bit of sheet wrapped around the middle to give the step down?
i dunno it might not work but just a thought

Well I can't weld ally or stainless so there it would fail on making one. I'll have a look at the Burton one and check my measuring. That just might do.