There might be a problem, as you see the exhaust vents out the bottom so you might want to cut a hole in the bottom of the armrest to allow air to vent. Also the intake is on the bottom so you have to make sure it is not snug against the armrest.
That's what I did. I oriented the Mini vertically. Cut a hole in the bottom for cables and exhaust. The intake vents on the bottom of the Mini are set back slightly so snugness isn't an issue. On a hot day I'll keep the armrest open and direct an AC vent towards it. No overheating issues in 3 years now.