I wanted some more space between my bed and shelf wall. Eventually I settled on shortening my bed by 20cm. My mattress is not normal at all, and will fit the container it is in.
To start the two uprights near where I need to cut get moved past the cut line, with enough space to get a hacksaw in there. Everything attached to the end part of the bed needed to move for the process.
It was a sweaty time trimming the top four boards.
After the trim I tacked the end board back on while trimming the platform.
half way cut.
rest of the way cut.
put the end back on
line the verticals up with the end of the platform, make sure they are plumb, and secure them.
using ropes, and a big clamp, force the top board into the right place, lined up, and tight against each other, and finally secure it in place.
fix up the rest of the top boards.
and the extras go back up.
finally, I put a lip on the edge to help keep the contents of the bed from falling off the end as easily.