Step 1: Materials
Some aluminium foil
Step 2: Assembly
Cut a ring from the cardboard tube, or otherwise make one. I found that a standard British toilet paper tube is just the right size. Pack the crayons in the tube, hexagonally.
Apply glue to the bottom of the crayon-pack, and work it down the cracks (e.g. with a pin or piece of wire) . The glue will hold the crayons together well enough, and secure them to the plinth. Add the crayons to the plinth and allow to dry. I used 2-part epoxy: strong & heat-resistant.
Step 3: The Melt
Then into a gentle oven for 15min.