不下于

不 = 丆 + 卜 : Bruno Bear (bu) is setting up a wind wheel (丆) with a voodoo staff (卜) in the space station's bathroom (Ø4). He also puts a "forbidden" sign (mnemonic symbol for 不) on it, to make clear that it may not (不) be stopped.

下 = 一 + 卜 : Mnemonic symbol: from the up/down logic, and a little bit the shape of the character, a ladder.

Xi Shi (xi) is on a retreat in the ashram. Using a voodoo staff (卜), she divined that next (下) week she'll have to play the flute (一) sixteen hours per day. Because she hates playing the flute, she climbs over the ashram's bathroom (a4) wall using a ladder (下) to escape.

于 = 亅 + 二 : Mnemonic symbol: prepositions (in, at, on, to, ...) are what glue the sentence together. Honey is as sticky as glue, so I'll represent 于 by a honey jar.

Yu the Great (于) was assigned to repair the electronics of the space station's kitchen (Ø2), but instead he was snacking honey from a honey jar (于). Unfortunately his capacitor (二) got stuck in the honey and now he has to use a crowbar (亅) to lever it out.