Probably your best bet is to purchase one of those
fake switchblade knives (or knife/comb combination)
from your local magic shop or novelty store. Then,
using paint, paper-mache, cardboard tubes,
stick-ons and your own imagination, disguise the
thing (without interfering with its mechanism) so it
looks less like a knife and more like a tiny
kaleidescope. The actors simply pretend to look
through the end of it -- too bad if they're disappointed
there's not actually a girly picture inside.