There is nothing worse that a leaking battery in your rack gear that has not only gone dead but taken PCB traces to the grave with it. I've seen plastic battery holders that are still on the PCB that "should" contain any acid that will leak from a dying battery, but I want the battery completely off the PCB. Not only that I want a battery that will last longer and be easier to change than a soldered on button battery. I use CR123 lithium batteries that are used for my alarm system sensors, I always have some around, and they are the correct voltage.
I purchased a bunch of CR123 battery holders and place them with Gorilla double side tape to an off PCB location in the rack unit. I've done this for many of my other rack mount effects and not just for the MP-1. Advantages are no chance of leaking battery acid on the PCB and dissolving traces, battery leakage will be contained in the battery holder, and CR123 have a much longer lifespan that a button battery. CR123's predicted lifespan in an alarm sensor which draws much more current than memory chips is 5 years. I am guessing that these could last a minimum of 10 years, their rated shelf life, but probably will go quite a bit longer. This is my MP-1 installation.