Tom. A few suggestions/comments:
1) From what I've read mast construction really does need to use a carbon skin and even then UD rather than twill. That said the stresses should be significantly reduced for a shorter mast...but I wouldn't risk using glass.
2) I think it would be much better if the finished mast could be dropped into and potted in the fuselage aperture..guess this depends on the size of the aperture?
3) some of the Alu foils seem to get away with a butt joint and 2 x 8mm screws for the mast to fuse connection. Not sure what the configuration is for your Moses? But if it is 2 x 8mm (or better), your idea of making a separate plug to go into the fuse with a flat top surface to butt against the mast might work, replacing the carbon rods with attachment screws??
4) if you are making a composite mast, i would use barrel nuts in the mast to give something solid to screw into.
5) This may be an option for you: http://www.thekitefoil.com/e-shop/!/Man ... 3D12622627
The sad bit is that even though they are only $55 the postage is likely to be the same
Depending on your moses screw sizes and spacing, you may be able to use 3) in conjunction with one of these?
For mast to plate connection you could use http://www.ketos-foil.com/en/kf-box/25- ... isser.html
and pot the mast into it..or possibly just butt the mast onto an aluminum plate and use the two 8mm screws..Zeeko seem to do this but they have 3 screws..Revhed has claimed success with two screws and a fillet of some exotic 'putty' - In this case I cant believe the fillet added much strength. In your case, with a reduced mast length, I would guess there is a good chance of success?
one of the advantages of the Al mast approach is you can cut it down to whatever length you like. I you want to know dimensions and screw spacing for the Manta mast, I can get these for you.
Food for thought..