so, here's an incredible cross sections scan of the v-150 cannon on hoth:

The cannons are absolute beasts. Don't get me wrong.

We don't actually know how big the thing is, at least not directly. We do have a scene with a golan df.9 turret (the stubby tower ones, not the dish turrets,) is next to it. those are 4 meters in height In the screen grab of the ion cannon. guestimating the height behind the snow drift, I make the ball 6 df.9's in diameter. 8, if I ignore the base and go with whats seen, and ignoring the actual barrel thingies. (that's another df.9.)

So for surface installation (Ball turret plus the reactor core that's under the ball,) I'd make it needing something between 48 to 64 meters in over all length, and 24 to 36 meters in cylindrical space.

this wouldn't be that hard to bolt on to something like a GR-75, which is 90 meters long, and mostly just cargo racks. Keep in mind they had to get all that equipment there in the first place, and these aren't the kind of things you're building in place, it might be broken down into some submodules, but you're not breaking them down into component parts.

if a GR-75 isn't large enough, (maybe because the reactor core isn't... a reactor core? and needs "base" power.) Then the Imperial Container Ship certainly is (and incredibly easy to steal.) You can actually build that thing out to something that resembles a mobile operations base, to be honest. (the internals of the cargo stack are all shirt-sleeve habitation, apparently,)

Both ships have an advantage here in that they would be rather easy to modify into hauling something other than containers, and including possibly something outsized like the ion cannon. basically becoming a system defense monitor (slow, big guns, and mobile only the sense that, sure, it technically can shuffle off.)

You could do a few things with it. use it concert in lightling raids- it doesn't come deep into the envelope, but it hits the biggies from range. Probably after a recon flight pops in and gives targets, it can jump in and start blasting with lighter things coming in to do whatever. fighters, bombers, etc.

alternatively, drop it in orbit and let it hang out where it can cover an escape and shuffle off on it's own. (bonus points for everything else being space-based and not planet-side.)