front derailleur clamp spacers

hey, next problem on new build, i currently have a shimano altus front derailleur, not the best but never caused me any probs so want to utilise it on frame. prob is new frame is 28.6mm and the clamp is bigger, 31 something mm i believe, i was given a deore spacer but it doesn't fit my altus clamp. how do you guys get round this? i have some gasket rubber, could that be cut down and put round frame and clamp front derailleur to that?

All ideas welcome, thanks