Just yesterday I was trying to figure out the same situation with cutting
black rubber heater hose to fit my tugboat's bow push bars. Couldn't get
a straight cut.
A friend watching me said, "Why don't you just slide a piece of round
doweling or a rake handle or something into the hose and then you can
clamp in down and, tracing a straight edge, make a good cut.
Well, I just happened to have a piece of round hardwood stock and the
hose slid on perfectly. Using a brand new, a sharp blade is a must for straight,
I was able to a get pretty good edge. I used sand paper to split the difference.
The other suggestions all work to, TBone. . .Michael in Anacortes, Washington