This is a common problem. I think a summary recap answer is warranted.
Proper Stopper Size and Style. There are dozens of styles. Hoods, drilled plug, molded silicone etc. Pick one you like, they all work just some easier than others. Most important get the right size for your carboy.
Hold it In. Tape, foil, plastic food wrap, champaign cork retainer wire, palet packaging wrap.
Reduce Pressure Don't over fill the blow off bottle. Couple inches is all you need. The depth of the sanitizer translates into psi the stopper has to hold in.
Clean and Dry Make sure the stopper and carboy neck are dry. Clean paper towels work well. Using a sheet that you unwrap to get to has worked well for me.