Well the middle seat is a waste of space for anyone over 3 years of age I think. As for the cubby box make one yourself if you can that way it will be exactly what you want and you'l have money for diesel, beer, food the list is endless.
To me its like having a garage.... all you put in it is crap unless you need a cup holder a place to hide a radio. They are expensive for what they are unless you can make one up.
My middle seat is out and I have 4 inches of crap filling the gap... I use the dash over the speedo as a cup holder.... takes 2 flask tops no problem
leave your centre seat frame in place and make yerself a cubby box that sits in the frame. Then keep the seat base in the back of your landy. You got the best of both worlds then cos you can switch to whichever you want in just a couple of mins.
I built one for a mini years ago, ply and vinyl and it cost bugger all-just make cardboard template first ply sides,wood reinforced and screwed and glued.
Vinyl or similar over it
I've found some pics. They aren't great. They do show my DIY cubby box and you can just about make out the seat back still in place in one of em.
It was held in place in the same way the seat base was. 2 steel dowels in the back of the cubby box and a steel bar across the front that located in the front clips.
My last landy was a 110 van. The rear was sectioned off with a wooden bulkhead. I couldn't afford to lose the seat but also wanted a cubby box. It was the only thing I could think of.