A sheet should do it.So I didn't think of this issue. Because the brooder house has wire sides they can see in and see their usual night roost. So at least Piglet is trying to break in from various sides.
I might cave and open the door.
This is all a bit distressing. I should have covered the whole thing with cardboard so they couldn't see it.
Gently put them on a roost in the main coop - all the Pentagonists together, and if you are able, tack up an old sheet to 'hide' their brooder hut.