Medium term repair for cracked shower pan
Materials Needed

A sheet of something to act as a patch (FRP, acrylic, etc.)
Good two-part epoxy
Sandpaper (150-220 grit, maybe)

Procedure

Cut a patch to the desired size. It should overlap the crack a fair bit to act as a gusset.
Trace the patch on the tub with a pencil. Optionally mask outside the patch area to protect that surface.
Scuff the patch area with the sandpaper to clean it and rough it up. Also scuff the patch sheet if it's not already rough.
Bond the patch to the tub with the epoxy. Finger out the epoxy that squeezes out to create a short taper and seal the edge.
Ease the edges of the patch with sandpaper to prevent injury.

You could also do an old airplane mechanic's trick to stop the crack from growing first: Drill a 1/8&quot; or 3/16&quot; hole at the ends of the crack. This distributes force over a wider area.