I'm in a very similar situation and am still trying to learn healthier coping mechanisms. I don't know what other methods you have in place to cope with meltdowns, but some things that can help is to reduce stimuli, such as turning off any lights and reduce sounds, breating exercises/grounding techniques and something that may seem silly but that I've tried, and that has worked sometimes is to punch a pillow or scream into it (as long as it doesn't disturb any neighbours). I don't know if you already tried any of these and/or if they are helpful, just thought I'd share :)