When I installed the glass cabinets in place of the open shelves it was clear there was still room for some extra shelving. Even though that was only 3 months ago the ever expanding size of the collection has meant that already I have made some changes to the layout and at the same time taken advantage of a few good opportunities to improve the area as well. Here's an update.
Fortunately New Zealand's eBay equivalent, TradeMe, came to the rescue and I found a company selling shelves for an incredible $2 each. They were a wholesaler who looked like they'd accidentally come into a load of shelving designed for bathrooms that they needed to get rid of. The shelves came in a rather random variety of sizes but would fit inside the existing cabinets. Even so they were not an exact fit and I'd have to find a way to hold them up, as they weren't wide enough to reach the front or far side.
The answer has come in the form of some tall shot glasses I found at KMart for only a few bucks. They are the perfect height to allow the shelf to rest on their top but still have enough room underneath to display models on the lower shelf. At right you can clearly see the shot glass holding up the shelf above. Being transparent they are not too obvious and are actually quite attractive.