Rear storage box

We finally have our 1987 Ivevo daily campervan on the road . We could really use some extra storage and the roof isn't suitable due to awning, solar panel, roof vents etc and am thinking about adding a large rear storage box. The van has a sturdy looking tow bar fitted if that could be of any use?
Am wondering if anyone here has experience of this in terms of getting one fabricated or purchased and what the regulations may be regarded size etc

We also have a Fianna backbox mounted on the bike rack. Is your Daily a coach built motorhome or a van conversion with two barn doors on the rear? If it is the latter you will not be able to open the doors unless the box can swing to the side, either mounted on one of the doors or mounted on a purposely made swinging bracket.

You have to be careful of the extra weight that you will be adding further back on the van. There is some formula that works out how that works. For example if you put it very far back it is like adding more weight than it actually is. Some of the more enlightened may be able to explain. When I pack the garage in our van I put the heavy stuff as far forward as possible to counter this.

Like, the moment I'd realised I'd put too much weight in the garage was when I tried to take that bend too fast, and some of my traction had disappeared?