If you have a VOM, remove the neg cable from the battery (it must have a charge). Set the VOM to the 12 volt DC range and hook it's positive lead to the negative battery cable end and the negative lead to a good ground on the engine. Make sure everything in the car is turned off and all doors etc closed so no interior lights or anything is on. If there is a current draw, you'll see it on the VOM. I think some components on the car ( it's computer, alternator, digital clock in the radio ) will draw a very small current when everything is off but certainly not enough to kill a good battery. If you see a current draw, start removing fuses one at a time until the draw stops. Whatever is killing the battery is located somewhere in the circuit served by that fuse. Plastic or metal battery trays is, as said, crap.