I have a few items (such as my Thumper in car charger outlet) in the back of my Puma which have velcro stuck to them. I lined the back out with marine carpet so the hook component of velcro's hook and loop sticks really well. See in the bottom right corner of the attached:
Attachment 148246
For the Gullwing, if it has a loop type lining (ala marine carpet or even the wide velcro strips) you could make up some angle brackets out of aluminium (a tee section would be best) to use as adjustable partitioning. I'd also consider shock cord secured at each end and have some open hooks at convenient spacing to hold stuff in. The success of the storage would largely depend on having suitable containers I think.

