No those dimensions were outside
Yeah youre better off with a single box, means less work for you dad
Your better off going with the port on one of the sides, its easier to make and wont make a big difference to SPL. Also you wont need as much port depth as the side of the box is one wall of the port if you get what i mean.
So yeah, that box is 5.28 cu.ft. before sub / port displacement (wood thickness already taken out to get that 5.28). Take out probably no more than 0.1cu.ft and thats what your left with. Using a slot port to get 33hz tuning (with that port width) you'll need a port depth of 36.5cm, take into account the baffle thickness and you get a port wall of 34cm. Now, your box is 450mm deep (internally), which means you can go to a maximum depth of 40-42cm max. 42cm only leaves a 5.5cm gap between the rear wall and the port end, but as its only for daily use it wont be noticeable (probably 0.3-0.4db difference).
If your dad cant round over the mouth of the port and the end of the port wall really well (like in that link from subwoofer builder), then i'd go with 6" PVC pipe. Just flare the ends like
this.
If you do go with a round port, my suggestion would be go 2 x 6" ports, one above the other in the centre of the box. It will give you more port area than the 3" x 15" slot, which is good for SPL purposes. To lower the frequency for music just plug one of the ports with a towel etc. That'll lower the tune to 28hz, haha.....thats a nice low tune anyway, but you can shorten the depth and up the tune if you want.