Measure your components and make sure they'll fit in the case. Print top, bottom, button. Fit electronics, test, and glue the top to the bottom. (By using superglue, prying it apart is pretty easy.)

I sanded the top/bottom and sprayed some plasti-dip to give it a semi-rubberized surface. (I think I liked the pre-sanded feel better.)

Code is for an ATtiny13 chip. The code builds to 717 bytes, which fits nicely. I simplified the circuit to only a 3V button battery, a switch, an IR LED, and the ATtiny13.

The push-switch is scavenged from a musical-greeting-card, as were the battery contacts.

Boosting to 6V doesn't increase the range all that much. As the battery can only source ~20mA, the addition of a capacitor might improve distance.

Fitting and hot gluing all the components takes some finagling; try not to burn yourself.