How do I specify bags in the management condition interface?

Having located the magma layer as well as a good source for sand, I’m eager to get glass production up and running, for trade and defense purposes.

In order to better automate things, I set up a glass furnace that doesn’t take general orders, but has a specific order to gather sand 10 times whenever there are less than 50 sand-bearing-items available.

However, in order to avoid running completely out of bags I also want to add a conditional not to start the job if there are less than 20 empty bags available, and this is where I run into trouble.

You can specify ’empty items’ as a trait, and ‘boxes and bags’ as a category, but that category includes wooden chests and rock or metal coffers, neither of which are suitable. On the other hand, material doesn’t have categories like ‘all leather’ or ‘all yarn’ but only specific ‘aardvark leather’ or ‘sheep yarn’ or ‘ramie cloth’.

This makes the latter test essentially useless since I have a different work order to make sure I always have at least 20 rock coffers available for when I need to create a new bunch of rooms.

Any suggestions?