Notice: Answer from a Belgian French perspective

This is a tricky one. If I were to see the picture you describe and the product was labelled as "sacoche" I would probably not find it too much off.
But if I had to choose only 1 word to describe it, I probably wouldn’t use "sacoche" but pochette, housse, étui, sac,…

Using more words you can also be more or less descriptive, and then refer to "sacoche":

Sacoche/Sac/Housse/Étui/Pochette bandoulière pour téléphone portable

"Bandoulière" designate the shoulder strap. "Porter quelquechose en bandoulière" means carrying it using the shoulder strap, usually across the body.

Sacoche in French Quebecois mean principaly the hand bag of a women.

It can also mean a object like a sac banane, but it got a strong connotation of a feminine’s article. Hearing someone to tell that to a man’s bag could make me think if he wanted to make a joke.