One thing nobody mentioned is the "Collisions" option.  Having never used it myself I don't understand the functionality but I think it may help with what you're trying to achieve.  At least, the "not passing through" part of it.  The flattening effect is something else, and probably best realized with morphs, as mentioned previously.

As you can gather from the posts here, there's no simple way to make an object "solid", ie, impenetrable.  Since they're just a bunch of numbers specifying vertices, there's no real way to do it.

Something just occurred to me...

The Cloth Room does treat objects as if they were solid.  With several limitations.

Again, I have no idea if it would work but I have a feeling the Cloth Room may be of use to flatten your slug against the window.  There may be a way to clothify the slug so that it looks flattened in places.

Coppula eam se non posit acceptera jocularum.

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