I'm in the process of building a cargo trailer. It's not a walk-in closet like you've got (I didn't want to tow a big box behind me), so it will not be as nice, but hopefully it will be functional. I'm calling it an "off-road camping cargo trailer" as it is pretty beefy and supposed to go anywhere the Jeep can go.
It's not just for scuba, it's for camping and general purpose use too, so it's not like I will leave dive gear in it all the time. I couldn't do that anyway because I don't think dive gear would like sitting outside all winter.
Trailer is currently at the powder coater's place being powder coated - but here's a picture of what I have so far. It has three heavy duty security boxes easily accessible (one of which will be set up as a galley), and several "inner chambers" - the main cargo area is accessible through a rear door. In these photos, the "skin" floors, doors and fenders are not shown. The main cargo bay has welded on d-rings (lots of them) for tying down the scuba tanks and whatever else I want to put on there.
I can't wait to finish this thing so I can use it for scuba and camping. It will become the permanent home for my camping gear - so I can just leave it all in there and go at a moment's notice. SCUBA gear will only go in there in preparation for a SCUBA trip