I assume that neither this nor the 40oz...
I assume that neither this nor the 40oz will fit my favorite splashguard (2 in. diameter)?
Correct, the opening of the Kleen Kanteen bottles is 1.75" They do manufacture a sport cap, which will help prevent spills, but unfortunately backcountry does not carry it.
I've found that you don't really need a splash guard, to be honest.