I bought a battery tester on amazon, similiar to this one: https://www.amazon.de/ChuYa-Analoger-Batterietester-Batteriepr%C3%BCfer-Knopfzellen/dp/B07NXXCHT6/ref=sr_1_2?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=1AAPWV5MS8M9Z&keywords=Tacklife+MBT01&qid=1706334483&sprefix=tacklife+mbt01%2Caps%2C206&sr=8-2
There are several of them which all look the same, so i expect also other battery tester to fit into the holder.
If you want to fix it more secure to the pegboard use my TPU clips: https://makerworld.com/en/models/47195#profileId-48898