Lots of bivy bag-like options, but a bivy bag is for surviving a storm, not for sleeping. One night in a good storm in one and you’ll see why. Condensation is an issue no matter the material. Putting in a long day on bike after a poor night of sleep sucks; not at all worth any weight savings . A bivy bag with even one pole at the shoulders makes it livable. (A stiff wire around your head doesn’t count.) Bug netting is mandatory.
My UL 1P tent weighs about what my bivy bag does, and it has tons of room for everything that needs to be dry, it also has a vestibule. I’ve spent over 60 nights in it, it kicks ass: https://www.nemoequipment.com/product/hornet/
Fred, it’s not hunting on a MTB ride, it’s using a MTB to hunt.