That's a nice setup. I was going to suggest a Blue Ridge Overland Gear cooking bag attached to that grid and then attach the spices with velcro strips but it looks like the metal tins idea worked out great.
Depending on where you live/travel and how often you go through them, those oils may go rancid quicker being in the back window of a rig like that. Just something to keep in mind. The spices probably won't like it much either.