LED properties for throwies
This product is a consumer liability. 
Preventing a short and subsequent exposure to rain or water is paramount.

Ignoring this risk outlined in the MSDS of any Lithium Manganese Diodxide battery and blocking any vents with sealant or epoxy is also a bad idea.

There are kits from China for under a dollar per CR2032 10mm diffused LED, but I have no experience with them so I won't recommend any.

Here are some guidelines: 


Energizer coin cells have approx 10 ohms internal resistance (IR) which rises quickly towards end of life. Panasonic appears to have a much higher IR (~100ohm) perhaps for safety reasons.
Diffused LED's would provide best display properties at all angles.
10mm LED's can be bought for &lt;25 cents but compare mcd levels with 5mm LED's.
Different colors have a nominal voltage that may exceed the 3V cell voltage which has a large positive temperature swing of +/-0.3V that may also force excess over or under brightness.  Close to 3.2V will last long but dimmer. Close to 2.8V will be brighter but last shorter.  
Expect maybe 50% of rated 225 ~ 250 mAh capacity when overdriving well beyond rated load.
Getting the right match of LED's to CR2032 is a matter of luck since there are so many variables uncontrolled.
use silver conductive epoxy for a reliable bond of leads on either side of coin cell, sparingly with excess lead length sheared off.
Rare earth magnets must be epoxied to case are very brittle so must be coated.