I had picked up a stacking storage thing from Target back around Halloween, for about $11. It has three medium sized trays, and a lid, and they all stack and latch together. Kind of a ghetto Systainer setup. But the footprint is about 6" x 10", and about 6" high.
The trays were the perfect size/volume for a full pack of Dominos (up to the 8mm size). I used that for a few weeks, and thought it was great.
But, then the green fever took hold of me, and all things must be grey and green [wink] , but having already purchased some of the bits, and already having several bags of Dominos I didn't want to buy the full $260 package. Also, I didn't feel a Sys2 was necessary for holding tenons (I don't have every size), and the bits can store in the Domino tray.
Soooo... short story long, I made a 1/4" ply insert, like the one that comes in the Domino case, but put it inside a Systainer 1. Good perfect size for my needs and storage setup.
Now, with all that said, there are some new and unopened Domino assortments that have been listed recently on Ebay and other online classified places. They seem to be going for about $200. If I had known about them prior to doing the tap-dance above, I would've picked up one in a second. For $260, the value is *barely* there, assuming you will use all sizes at some point. For $200, the value is definitely there, as it equals the cost of the cutters and Systainer alone... So at that price, the tenons are basically free.
Addressing your pseudo-question about fitting a small assortment of Domino into the main Domino case... fuggedabout it. I can't believe how much volume a bag of tenons takes up (I'm talking about the "small" bags, for $15.50). IIRC it's about the same volume as a small cantaloupe melon... You *might* be able to get one bag of 5mm in there, but no chance for two, or the 8mm/10mm.