Genuine Samsung 30T/40T/48G/50E and LG M50 fits the Convoy S21A.
iJoy 3750mAh 21700, Sofirn 4000mAh 21700 and Liitokala Lii-40A 4000mAh 21700 battery diameter is slightly too big and won’t fit the S21A battery tube.
I’ve read that some people have some luck inserting the Lii-40A 21700 from the head end (instead of tailcap end) of the 21A battery tube though.