Based on the etymology, I suspect the singular of chassis is case. Nobody uses it like that. But you would perhaps use chassis when it is full of parts or as an item that could be full of parts and case when empty or the parts don't matter.

Surprisingly enough, despite nobody using it like this, it works well, except for cars, where the singular form would be frame(car case just does not work), which does not share the same root and as such is a bit disappointing.

And what is also disapointing, despite the word already being plural(a french spelling of latin pluralization rules for case, use the latin soft c for case and add an i to pluralize, under french rules "is", the s is silent), When there are several chassis, we use the english pluralization rules and add an s. And because there is already an s we just add an s sound.