The one that I remember from the high school production of this show that I was in ages ago was just a plywood box, much like a plain coffin that is wider at the shoulders than at the feet, then narrows again above the shoulders.  It was painted to look like King Tut's mask and gear.  Make sure that it will close securely and when they carry it off with the actress inside, they just tip it and lift.  I can't remember very well...would it be possible to put it on a furniture moving dolly or does it have to be carried off?

To save on weight, it is possible to build the
sarcophagus with the bottom and the two endcaps
(head & foot) made of sturdy plywood while the sides
and lid are built of foam core or corrugated
cardboard.

Cut the bottom shape out of (at least 1/2", preferably
3/4") plywood. Screw & glue the endcaps securely to
this. This will give the structure some stability and
will serve as the lifting points.

The sides are attached using staples, glue, screws,
chewing gum or whatever works for your situation.

The advantage of making sides and lid out of foam
not only saves weight, but the foam can also be
built-up with extra pieces of foam and carved with
Egyptian symbols and decorations, creating a relief.

Latex caulk in the hands of a skilled craftsman can
create further decorations. One would handle the
caulk gun as if they were decorating a cake. Be sure
to allow the creation ample time to dry before
painting.

A plastic Halloween face-mask glued to the lid and
painted over saves the time and effort of carving a
Pharoah's face.   Caulking or extra pieces of foam
glued around the face cover the seams and create
the head-dress.

Give the whole thing a thorough coat of gold paint.
Highlight the decorations with black enamel or other
colors. Even glass beads or fake jewels can be
encrusted with hot-melt glue.

Just remind your actors to not sit on the thing as it
will crush. You can get as many hands as needed to
carry it if they all lift from the bottom.

PS: Cast a skinny actress.