Using sublimation on a Poly hive?

I use Swienty with floors that have half circle shaped holes. I intend to block off the Dow draft with thick card, rest a slate on top, then bing up spaces with towels (after putting vaporiser in).

I use some roof slates. They are thin and flat. I slide them under the hive but on the stand. Wrap a towel round...no problem. The Beehaus are trickier. I use an upturned wooden brood box with a slate on it with the towel around the edge...seems to work. Ideally...I would like an eke with some ply as a bottom to it and rest that on the up turned box. Must try and sort that before the summer.