Want to find a bag, preferably messenger, that has a compartment (like for a laptop, for example) that can fit something 50cm by 30cm.
That’s big! For the purpose of a 21” monitor, which I can get under those measurements with a slim bezelless design. Needing a portable display, and not liking any options out there.
A messenger bag that car carry a 21 inch monitor would be uncomfy as hell. Don't know why you would want a messenger bag specifically. Come to think of it, a messenger bag that can fit a 21 inch monitor is a pretty ridiculous/stupid idea
Honestly I'd just get a suitcase. I do that every now and then when I need to lug around large briefs around the city. Seems much easier to carry, most have 2 compartments too + plenty of room for your other gear.
21inch? What is this a monitor for ants?
Get a projector lol
Why on earth would you want this ?
Might want to look up 'laptops' in your Encyclopedia Britannica set (or Encarta if you are up with the times).
I suggest the original monitor packaging with handle or Ikea bag.
Specifically, particularly, none may be quite what you're after, but bags for the Apple iMac exist.
I've seen a couple of 27 inch iMac's being carried over the shoulder. It's a look.
https://www.google.com.au/search?q=imac+carry+bag
Maybe something for the 21.5 or 24 iMac in mind will work for you.
https://www.google.com.au/search?q=carry+case+for+imac+21.5+…
plastic bag from supermarket.
You can buy some humour off ebay.