A "picture" project.

Two slightly different diameter O-rings are required: the first should allow a CR2032 battery to be inserted into the inner part and the second O-ring should have an outside diameter just slightly bigger than the battery diameter. I bought a box of imported (cheap) o-rings from an online store for a couple of buck$.

I used a wrap of Kapton tape to secure the battery and o-rings such that the positive lead to the LED (anode) was firmly bound to the + of the battery case. The negative lead is pulled over the top of the second (smaller) o-ring and bent to ensure that there is an airgap that will be closed when the plastic case is squeezed.

I used a yellow-ish LED but I tried several from the reuse box before I found one with the best performance for the CR2032 battery. Depending on your choice of LED, you may have to insert a current-limiting resistor to protect the LED as Ohm's Law is still valid.