Should I build a shield to protect building wire installed inside cabinet?
This is perfectly fine and quite safe. The metal box could even be sitting out in the open and would be just fine. I have a similar setup in one of my kitchen cabinets. The only concerns I know of are:


Drawers - This does not appear to be the case here at all, but if you have a drawer that can bump into a cable when it is opening/closing then that would be a problem.
Loose Cable - Also not a problem here. Even when it is a problem, it is usually not a problem to fix. Cables basically need to be short (as is the case here - just a couple of inches extra which is good in case anything ever needs to be changed) or secure and out-of-the-way (and this is clearly out of the way).