I used 3M 08090 spray adhesive. It's a heavy duty glue designed for exterior vinyl tops. "Normal" headliner glue will not hold up under the heat since the headliner is glued directly to the underside of the roof panel which experiences direct sun (and extreme heat). Countless people on the DeTomaso mail list can tell you about their re-glued headliners falling after one Summer of driving. 3M Super Trim Adhesive 8090 is available from numerous online sources, as well as your local Napa Auto Parts store.
Here's how I did it: https://thepcnc.weebly.com/headliner.html
+1 on the magnetic sheet idea! I really like that approach, and the ease of adjustability/reattachment if you don't get it positioned just right on the first go-around.