I've just had to resort to the ancient technique as there is no power
in the house I'm working on.

Cut a piece of floorboard a little longer than your work-bench. Saw
two perpendicular lines either end of the board, a few inches in from
the stool. Point the saw toward the centre of the board at the
required angles (45 degrees.)

Scribe a line parallel to the top of the board. Put a baton on this as
a ledge to support the work and there you have a safe, cheap mitre
saw. The two cuts want to be deep enough to give the saw guidance
before you hit the work-piece. If you are cutting long stuff put the
ledge at an ideal height to something that will support the other end.
(Or make two saw benches.)

The simplest way to make another bench is to cut an old door in half
and use it's hinges to join them and a piece of rope or a baton to
stop them opening too far.

Always use a new saw as a used one tends to cut off-line. Unless it's
a Jack saw. I never got one of them that cut straight. I think it was
the supplier treading on them so he could sell more. Get a Sandvik (or
Bacho) one.