One of the bolts broke, due to corrosion. $23 for a new one from Dodge, so I'd like to just buy some type of generic bolt. Trouble is, I come up with an off size when measuring. I realize it is metric, but its still an odd size. The female "nut" in the baattery tray is still good so I need the threads to match that. Dealership computer just lists it as a bolt, without dimensions. Does anyone know the proper size of this bolt, so I could order one from the local fastener store?

 

Loose the whole mount. They go out in couple of seasons due to corrosion like you said.
Get a single ratcheting strap and wrap it around the battery and the tray.
I got this setup on all my vehicles taking out the battery is super easy. Just open up the strap.
I'll post some pictures as soon as I get to it.
Hope this helps.