How to insert a panel in a tray to avoid touching edge and leaking: Aprilaire 600M humidifier?

When the panel was leaning against the tray, water leaked onto the tray edge (and then the floor) – see How to troubleshoot a leak in an Aprilaire humidifier? or the first photo below.

When I put in a shim, the leak stopped. (See second photo below.)

I do not see a method for inserting the panel in the tray so it does not touch the edge.

What am I missing?

UPDATE
I found an Aprilaire video. There is not such technique. Perhaps my tray has warped.
https://vimeo.com/115587899

Thanks.