Battery we are referring to are from "Jims Battery" cases with a battery inside. So when it fails, we are trying to figure out an easy way to change it out the inner battery. I assume all the hardware to hook them up are inside. So this would be a no mess situation like the one you are referring to. I totally agree, cleaning out an exiting battery is a mess and lots of work.
The hardware is there, but you will find the cables are too short. Best to get a longer cable and cut to size. There isn't much room in the case to fit your hand in there and make the connection to the battery.