insulation, how to secure?

We are restoring a '67 Safari and have most of the interior skin off. For the rear fiberglass piece, we took it down to repair a PO's attempt to add a bath vent as well as to upgrade wiring and insulation. Now we are ready to put this fiberglass piece back in place but how do we secure insulation in place? There are no ribs in the upper portion. We want to use rock wool batts. Do we try to tape them in place? Adhesive? Do we tape or glue them to the fiberglass shell and hope they stay in place. We have some good gaffers tape for securing electrical wiring if that would work.

Thanks folks!

Clueless Jim