Reddit mentions of HobbyKing XT60 Charged/Discharged Battery Indicator Caps (5 Pairs)

I've got these rubber ones which are great, too

You could use an xt60 female and a male/male adapter on the charger instead to mate the two female ends.. safer but still prone to water damage etc.

Ideally you could use xt60 caps like this

https://www.amazon.com/gp/aw/d/B01FUOFZE6/

Though personally I just use a piece of black electrical tape on the tip when not in use.