I have just cut my 2019 Slingshot mast down by 10cm,
engineering background many years ago made it stress free using a top quality full size hacksaw, correct hacksaw blade and good slow steady firm cutting action,
The mast "waste" end, held in a vice with mast supported on level plywood,
start at the thick edge with a fine black line from edge to edge to help you guide the blade, as you get near the thin trailing edge,approx 5-8 mm away, turn the mast around, and then using a junior hacksaw join the cuts together,
smooth off the cut edges, clean and smooth off the internal cavity edges with small files,
the holes to be tapped need a 30-40 deg chamfer, use a big drill bit to do this, or a countersinking tool is best if you have one, hold the drill bit in your hand as you do it, don't use a power tool.
Use the original uncut surface on the waste bit of mast to check for the cut surface being level, butt the surfaces together and hold against strong light,
I was very lucky as I was mega careful when cutting, and did not need to make any adjustments, that must be a first for me, don't expect to be so lucky!
Mix up a generous amount of quick setting 2 part epoxy resin and quickly spread it liberally just inside the now exposed cavities, (but not the holes that need tapping)
use a flat block of plywood to place on top of the mast, then quickly turn the mast upside down and place it securely on the floor, supported so the resin runs back down the cavities to plug the holes, when the resin has turned thick and very tacky, then use a stanley knife to separate the wood block from the nearly set resin, then quickly push down and smooth off the little semi cured plugs of resin with your fingers, leave to set and clean the surfaces.
(sealing the end with tape will work as well, and do the same thing, then turn the mast upside down as above)
only the tapping left to do now.
But remember not to cut the thread dry, use WD40, and lots of it, don't allow the cutting noise to become destructive,listen for a smooth cutting sound as soon a creak is heard then reverse the tap back a bit and flood with WD40, so easy to ruin the threads if you just attack it with a dry tapping tool.