Firstly, you must bring your substrate to field moisture BEFORE you spawn with what ever you are using as spawn, otherwise you play a game of catchup that you cannot win. Secondly, the more you handle your substrate after it has been set (spawned, containerized and the like) the more you plead for contamination, not only on the top, but the sides and the interior itself. Nothing worse than encountering trich inside your substrate with no evidence otherwise (it happens).
Don't worry about the walls of your container get a hydrometer. You want to try to avoid condensation, difficult I know as it happens with temperature fluctuations but try anyway. One does not usually "dunk" bulk substrates unless they are cellulose based - that is blocks of innoculated sawdust or sawdust and bran, P. Cub is not something that grows well if at all on such substrates.
Dunking is for small cakes that have lots of surface area to mass and prone to lose moisture. Another plus for casing is that the "external" moisture from the casing can be adjusted far more readily than a bulk substrate that may well have suffered overgrowth that keeps the substrate from absorbing moisture anyway,