I've read that the flow is at least somewhat dependent on low levels of epinephrine during non-REM sleep - and elsewhere I've read that brain derrived phosphadytlserine lowers both cortisol and epinephrine* so I can't help but wonder if phosphadytlserine might provide some benefit. (disclaimer: I'm not a doctor but have found it useful as a supplement when I'm jet lagged, trying to go to sleep when my internal clock is telling me it's *not* bedtime.)
Also, of all the 'glymphatic flow' papers I've read, I don't remember even one with actually-sleeping human subjects - and I can't find one now.. If anyone's found one with *human* subjects actually *sleeping* I'd be beyond curious to read it.

*That study linking phosphadytlserine and epinephrine was with *eight* subjects, *not* sleeping. Apply loads of salt - or potassium chloride if you have HBP :-).