Welcome, Daifpop,

I'm new myself, but thought I might have a useful suggestion for your problem of not being able to keep your eyes closed during your session with DMT.

I don't know if you tried this already, but get and wear a sleeping mask the next time you journey with DMT, the ones that people wear to bed at night, they usually block out any light. So if you open your eyes, it won't disrupt your concentration or experience.

Also try some meditating with closed eyes beforehand in a quiet, darkened room can really help you focus and can help make it easier to keep your eyes closed while exploring inner space. There are different methods that you can find via google, use the one that seems the best match for you.