#EDIT//: To all the parts geniouses:

Trying use a RDM extension (my only decent extension) to convert an old ~400cm mast for use with a 4.7

Don't wanna buy an SDM extension (plus the mast is cut down so wont fit anyway) if I can get away with it, as will end up buying a RDM 400 when money permits and a SDM will be useless surplus

So how do I adapt a RDM extension to a cut down (narrower) SDM mast?

Been trying a compressed foam/gaff tape wrap around extension and also to stay inside mast

Any ideas?

Buy an SDM extension, then sell it once get new gear!

use the mast upside down that way the extension will fit easily in the tip of the mast

The other thing I failed to mention: this mast isn't proper standard SDM, as it has been cut down already (i.e: doubt a SDM extension will fit, prob between SDM and RDm would work)

So i'll take it no one has done this?

I don't think wrapping it in anything soft like gaffa tape will do the job. You have to stop the mast deforming where it accepts loads. You have to have a reasonably snug fit and a hard non deforming shim to do it acceptably. Maybe you could find a bit of ally tube that is close to a snug fit in the mast. The fit between the extension and the tube is less critical. Gaffa n bandaids might be ok for that.