Aftermarket support for the 42RLE transmission is not great but there are a few pans out there with drain plugs... YourCovers.com 42RLE Transmission Pan
But for those pan's exhorbitant costs, I would just drop the OE pan and have a welder weld a drain plug bung into it. I welded a bung into my pan for a temp sender and it only took a few minutes.
Or buy Mopar's new version of that pan which is slightly deeper in one location to help reduce the likelyhood of the tranny pickup sucking air while you're climbing a steep obstacle. Then you can take it to a local welder to install the drain plug you can find from places like Summit Racing
These are Mopar part numbers...
Pan: 52852912AC $7.50
Magnet: 446728 $2.45
Filter: 5282913AB $19.00
As you can see a new Mopar pan is cheap and a weld-in bung and drain plug from Summit Racing is probably under $10.