Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4324082.
<h1>MT99 Multimeter battery mod</h1>This is a back cover replacement for Mustool MT99 multimeter (the MT77 and MT99PRO models are similar).
I wasn't too happy about the battery life given by the two coin batteries so I've made a case for a battery mod. It houses a battery, power charger and voltage booster if needed.
The frame has space for any battery up to 47 mm X 47 mm x 8mm thickness, when using the covered version. I've included a second model (edge), as a variant if you plan to use the original back cover. It has the advantage of about 13 mm thickness clearance for the battery.
You will need some supports for the Usb cable port, depending on the print position.
I've included some cylindrical geometry supports at the corners, those align with the back screws. It would be easier to drill the holes without hitting just the infill.
More details can be found on the instructable link.