In building 10-ft OD rocket booster o-rings, we used a brand new x-acto blade each time. Rubber is abrasive... If you google 'McMaster-Carr', they have several actual kits for building o-rings this way, including a square-cutting fixture, single-edge razor blades, superglue and a few feet of o-ring stock. Making the ends absolutely square is important to avoid a twist in the glued part. Practice a few times before gluing an expensive part. You can often cut the thing apart ONCE and redo things.
Fabricated rubber pieces that are used as stationary seals work well. For o-rings that have to move and seal, it's trickier because the glued point is less flexible than the rest. I once modified a Camaro clutch master cylinder with a glued-together secondary o-ring in a piston slot I machined myself, because the piston was an odd size and the OEM seal was an inadequate cup type- just like the Pantera. Worked fine at a clutch's low operating pressure.