I don’t know if this is the correct place to post this but I’m looking for a briefcase type bag that can hold my 13” linking rings and be stable when set on a floor or a chair. Something like Jeff McBride uses in his commando act on Magic on Stage Vol. 1 which is exactly what I am trying to accomplish, a grab and go bag and work directly out of it.

I was going to suggest a very inexpensive case from Home Depot.
It is aluminum clad 13" x 18" x 6". Unfortunately it will not quite accomodate 13" rings in the lid flap. Mine has three 12" rings which fit nicely.

My son gave me one years ago and it holds tons of close up stuff in the adjustable compartments.

You might find a suitable Halliburton case on eBay for around $100. Good protection although some folks think the brushed aluminum finish make it more prone to theft.