Have you tried adding the fourth piece (round piece, sorry don't know the name)?
It would look like the shield has a big circle in the middle
For an item to be accepted by a customer you must see a notification in the bottom corner of the screen saying "You created (item)"
There's 5 ingots worth of parts for a shield, the boss, the two handles, and the rim
I don't know entirely, but Try not to create a wood piece and/or combine the wood piece and a rim, and then save and reload - at minimum, the wood piece will break and be unusable, and I dunno on the combined rim/wood, Afaik, a fully assembled shield will be fine