Bun for "extra large" burger

I've had a request to make an extra-large hamburger; in the order of
4-5 pounds in a single patty.
Plan to use good ground chuck, cook it on a grill, and served with all
the condiments on top.
The problem is a large bun.
I've got a good mixer, and a thick baking stone for this project. Most
flours are available. I'm trying to avoid a bread that goes
"thud" (heavy texture), or one that would fall apart for being too
fluffy.  It'll be cut into 5-6 pieces after presentation.

Thank you for any ideas.
Pierre