Trouble with Tribbles (tradeskill combines)

[Asylum]

If possible, I'd like to have a single, no-fail item combine box that accepts any two of four possible items, each combination resulting in a different output item and not destroy the box. Item #s are below (server-specific).

box: 6688

items to be combined: 6596, 6598, 6599, 6632

output items from combines:
6596 + 6598 = 6634
6596 + 6599 = 6643
6596 + 6632 = 6655
6598 + 6599 = 6659
6598 + 6632 = 6660
6599 + 6532 = 6666

I'm having three issues creating this:
1) "You cannot combine these items in this container type!" warning, most certainly from me having an incorrect setting somewhere in the item editor for these items/box or in the recipes listings.
2) Having multiple items on your cursor is always a bug with the server, so returning the output item plus the box could trigger this bug as well.
3) How can I create multiple acceptable pairs of items in the recipe using the same container?

If you have an easier way to let players combine these items on the fly (not requiring an NPC or tradeskill station) I'd be interested in hearing your comments.

Thank you.