Rolf,
I have made a pin with 12mm threaded rod and a nut.
Screw the nut on. weld it in place and then grind a point of the rod. Put this through a stainless plate of the beam and bolt it in place from below. I you use a 25mm al tube with 3mm wall for the compression post you will need a hole in the beam that will be around the same size as the socket needed to do up the nut. So your mast pin transfers load directly to the compression post.

Now the fitting on the mast. Make friends with someone who owns a lathe.

Also as far as the mast base is concerned you should be able to make a mould with a mast off cut. Wrap something around the section that is say 5mm thick and make a mould say 50mm deep. lay into the mould carbon and kelvar apply a vacuum and weight until resin goes off. While still green sit the mast on top of the layup and lay in more to make a step that will locate base on the mast. So the outside edge would be 10mm thick while the centre would be maybe 20mm thick.

Also you could put a fair bit of glass in the centre to save cost of carbon especially aroun the edges keeping glass between the carbon and aluminium.