Is there a full-size closed headphone (circumaural or supraaural) as comfortable as Bose under $100
Not really that I'm aware of. There is, but it requires some modification. What I did was take the Beyer Dynamic DT-235 and replace the pads with those of the Maxell DHP-II ($30-$40). You now have converted the DT-235 to a circumaural headphone and it's probably the most comfortable headphone ever made. There is ZERO clamping force and the pads feel just like the Triports.
I was just going to say get the DHP-II but those are semi-open. They might have enough isolation to make them sound closed. On the sides there is a small grill with foam behind it. If it wasn't for that they'd be closed. Those have memory foam for the pads.
I use both these headphones and it's hard to say which sounds the best.