The various international advisory and governing bodies are debating "lithium" batteries all the time. "Safe" is a relative word and after every fire they are restricting them more. If this is general aviation and you don't want a fire while you are flying, out the radio in something like a metal ammunition can so it is confined in flight. For one small battery that should be enough. If you are flying commercial...the airline will probably consider it safe no matter where it is flying, but again, in a sealed (air tight) metal container is probably all the added safety you can have.
Traveling with the battery ~halfway discharged also adds to safety, but of course that's a problem if you need to use it.
A 30 caliber (308) surplus ammo can happens to make a generally good container for one h/t, one charging base, and accessories, whether you're traveling or not. FWIW.
And since the thugs working for the TSA are often a bit on edge and not the sharpest minds...If you do use an ammo can, paint it some other color, nothing "military", and consider sticking on some labels like "Fragile, radio equipment" something that helps them understand why the X-ray is showing wires and things inside. Wires scare them.