We're both 6'2" and the saggy headliner was driving me crazy. Not only had the fabric separated, but the cardboard had also warped from a previous leak.
On the previous build we had no board and had to make one from scratch. I really didn't want to have to do that so I looked into alternative options. People have mentioned a marginal success rate with getting it wet and forcing it flat for a few days. I wasn't keen on the idea because if any moisture got back in it would just warp again. I called our local upholstery shop and he recommended using resin and fiberglass to reinforce it. Seemed worth a shot?
After laying a layer of resin and fiberglass on the back side of the board we sandwiched it between two layers of Melamine coated in a thick layer of carnauba car wax. We screwed the two boards together to force it flat and left it overnight.
When we opened it up the next morning it was almost perfectly flat. We didn't do too thick of a layer of fiberglass so it seems like it will still be able to bed slightly to fit the subtle curve in the roof.
The fabric is arriving today so I'm hoping to have them installed again by this evening!