Have noticed a few posts recently for syncro mirror top gaskets and the fact that they are impossible to get anymore.
Have eventually got round to fitting the pair of mirrors that I purchased at SYNCRO 25 ................. yes they have been in the garage that long

I did happen to have one top gasket with them and have made a pair of gaskets ( please note they are handed), using an old innertube and some 10mm rubber matting.
Simply template the top hinge to the corner with the inner tube , easily cut with a pair of scissors. Mark out the screw holes and punch them out with a piece brake line or 8mm pipe on the back of a workshop vice. For the wedge shaped guide, to keep the hinge square with the bottom hinge and accept the third fixing I simply cut a 10mm rubber matting, please note this is wedged on two dimensions to accept the edge of the door framing.
These are stuck together with a waterproof glue........not superglue! left to set and drilled in the appropriate position.
Hope this helps someone to get their mirrors quicker than I did

Ady