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Level IV plate

Ceramic, composite and poly plates cannot be altered in any way. It may be possible to grind a steel plate to size, given proper cooling, but would most certainly void any warranty. I would either buy another carrier or seek to get it exchanged,
 
Not sure which plate you have, but all of mine are ceramic and have a kevlar cover, altering them wouldn't work. If you have the titanium plate, I'm sure you could trim it, but I wouldn't be altering anything that is used to stop rifle rounds. DIY and body armour shouldn't really be in the same sentence, but I guess do what you feel comfortable with.
 
Get a seamstress,parachute rigger, with a heavy duty sewing machine and attach HD elastic tape inside the binding tape at the edge. Or buy a different plate or carrier.

I'm guessing a waterjet would cut ceramic,titanium,kevlar and any combination thereof.Likely do a good job too. But installing a extension of elastic material at an edge would be the easiest and cheapest.