Burn/melt options...
Being a sailor and diver I have three melt tools that get a lot of use. A round pencil style soldier iron, a soldier iron with a flat blade, and a larger line cutting melter for large lines. I routinely melt/burn holes into gear, webbing, etc. This is for mounting threaded fasteners, or lacing. Before all the new manufactured equipment, we used to custom make our own slings, harnesses, weight suspender rigs, floats, lift lines and straps, etc. Generally you do not need any grommets since the melt creates a nice hard border. Burning/melting is the cleanest method. Don't cut since material will fray.
Sounds like you are a BC/jacket user, so as long as you don't encroach on your air cells or bladders, mount where you can easily reach. But be aware of potentail snag risk, weeds, kelp, lines, monofilament, etc. I prefer to have a small knife on my shoulder straps, right in front easy to access. I have this position on my BC and BP rigs. Snips on the belt line.
Also with my past experience, which includes air rescue, and life raft preparation, having a knife monuted on your dive rig is great, but also have one that goes with you, if you needed to do an equipment ditch, or whatever, a cutting instrument should be on your person. All of my suits have a protected pocket and cover flap for a backup.