I’ve been really happy with this: https://attaquercycling.com/collections/saddle-bags-pocket-pouches/products/adventure-handlebar-bag-camo
I’ve used on MTB and on road and with the attachment points and straps, it stays locked in place and doesn’t bounce up and down. Only thing you’ll have to manage is the headunit location, since it can prevent you from pulling up the flap to open it depending on where you have it placed.