I still have found over the years owning Cadillacs and Buicks,and you know how many weather
strip rubbers they have, that the best way is still by hand with a pair of those cheap plastic
snap of blades is best.
1--Use new blades every time
2--Available in all hardware stores
3--use a good flat wooden block on bottom sos handle clears bench top
4--When cutting slide blade forward and backward lightly, let the blade do the cutting, parallel.
5--depending on the shape and manufactures brand ,vary with rubber hardness.
6--The cut will come off straight and clean.
7--Sorry this example is just a garage door bottom seal.