I just had a thought concerning the artillerist. The artillerist feature allows you to determine the cannons appearance. If one plays a gnomish artificer with height between 2 and 3 feet, and creates a cannon in the form of, let's say a tank, between 3 and 4 feet, then casts enlarge on it, that artificer could reasonably climb inside and attack from full cover through peep holes.

Or a healy mech (imagine a medic protectron) with a seat for the gnome to sit inside.

I'm going to say no.  The Arcane Cannon is still a small-sized object.  That means that it's not going to be big enough for a small-sized creature to hide inside it regardless of what it looks like.

If you create a Small cannon and cast Enlarge/Reduce to bump it up to Medium? I can see mounting it in combat, but I wouldn't think it reasonable to say that it's big enough to provide full cover. I could probably be convinced of half cover, maybe. Medium is the size of most humanoids and even with a very large human or orc I don't see there being enough mass to qualify as more than like ... a motorcycle-ish mount.

That was one change that I was sad about from UA to published Artificer. I really wanted an Eldritch Cannon mount without having to use up a Concentration slot on it.

even if it did work, if your person inside is under total cover, its completely concealed by an obstacle and couldn't attack either (imo, if enemies can't see it, it can't see enemies)....imo, you're talking about three-quarters cover