FormerlyHemlock
Hero
The enterprising binder brings along a buddy who knows hold monster. True it doesn't work on undead, but it does work on outsiders (there is even a warlock invocation that references it). Conveniently a 5th level spell, too. Holds the outsider, while the binder casts magic circle. Binding follows. Oh, I see the problem. In your white room scenario, the wizard is alone. I am pretty sure there are no shortage of cautionary tales about solo wizards trying to bind demons. Real friends don't let friends bind solo.
Getting Hold Monster to last for ten rounds is pretty tough. Wall of Force would be a better bet if you want to capture demons in the wild.
Wizard 1: Wall of Force.
Wizard 2: Begins casting Magic Circle.
Then either wizard can cast Planar Binding.
I had considered this, but do you really think the target is gonna fail its save to shrug off Hold Monster every round for the spell's entire duration? As you said, Hold Monster is a 5th level spell, so even a warlock, being able to spend all their slots as 5th level, only gets 2 tries. Maybe substitute Hypnotic Pattern for Hold Monster, as it only allows 1 chance to save?
Or you could just beat the demons unconscious with nonlethal damage.
