The slot for the mast starts approximately 16in. above the boom. Lengthening the slot is absolutely required.
Important considerations of any improvement are durability and more importantly- thou shalt do no harm- if the extension causing snagging or other issues it's basically not worth doing. Additionally, easy removal (i.e. tool-less) while on the water would be beneficial, but possibly not necessary since there should still be room at the bottom to release a stopper and allow removal of slugs. Removal of the sail may not be the only concern though, if the backup sail is still missing slugs, or the need to bend a storm sail ever arises.
I have some aluminum channel from the garbage that I think might make for the perfect solution. Basic idea- two 20“ long pieces of similar thickness to current channel tongues bolted to four bent stainless steel sheet mount bent to match the angle of the built-in slot tongues, which are then mounted to the mast with either sheet metal screws, or some other quick release fastener. The slats should have counter-sunk stainless screws with heads flush with inside to reduce chance of snags.
Another possible mount is right angle brackets on the mast, slots cut in stainless sheet pieces to allow fitting a pin and ring vertically through a hole in the angle bracket. Another- bolts fit into threaded holes on the mast, hole drilled through the bolt to allow a ring. That might be the best, will try to make some drawings and photograph them later.
Of course after more reading, it seems this is essentially a common problem, and I see a couple of the same solutions at this link. Now that I know the opening to the slot is often called a “mast gate”, there's a lot of info to be found online.
http://www.rigrite.com/Spars/MetalMast_Spars/MetalMast_Mainsail_Gates.html#MM%20Thumb%20Screw%20Type%20Sail%20Gates%20&%20Stops - just noted for the convenience of the thumb screws
“Jacklines are lines that pass through the slide, allowing some distance between the slide and the shackle attached to the luff of the sail. When the sail is raised completely, the jackline is taught. When the sail is down for reefing, the jackline is loose.” Jacklines for the first 3 or 4 slugs may be an option, though the sail stack will still likely sit too high on the boom.
Sail height is 7.5m (295 in.)
Recommended spacing is 26-30in. - which means 10-15 slugs total. At a height of 1/2in. per slug, stack height would still only be 7.5” with 15 slugs. Going with small folds/slug spacing might be nice for tighter folds. At 20 slugs, stack height is 10“ (25cm).
Cost: 24dkk for pack of two, cheaper online
Another consideration, will the new space introduced by the slugs cause an issue with the halyard- will it pull the head diagonally in towards the mast?
http://bbs.trailersailor.com/forums/macgregorarchives2/index.cgi/noframes/read/29407 “The easiest is to use the screw on shackles. Since they loop over the bolt rope, there is no stress that would cause the sail to tear - the screw only serves to keep it closed over the rope.”