Perhaps try opening your left hand from a fist into a C shape that you drop into place on the neck. with the thumb tracking up and down the middle of the back of the neck. The fingers on the strings will be arched to about the same extent as they are when resting on a piano keyboard and there will be plenty of "daylight" between the palm and the side of the neck( enough to poke your bow through.
Observe that as your fist opens your thumb is not quite opposite the second finger when just able to accommodate the thickness of the neck and fingerboard, and your thumb is still slightly bent.
Observe also that there is a slight flat area between the centre of your finger print and the end of your finger that is possibly bisected by the string groove when your finger is arched in the "piano" shape above.
These are my starting points for teaching good left hand shape on the neck, and might need to be modified slightly for individual hand shapes and finger lengths.
Another way is to hang your left hand, loosely cupped, by your side then raise it as naturally as possible to the neck, opening up the thumb to accommodate the thickness/ If you are sitting comfortable in relation to the bass then the shape of your left arm will flow naturally from elbow to fingertips across a fairly flat wrist. This works for the top three strings however the wrist has to pronate a little ( or a lot !!) for E string notes.
To minimise the pronation I use a "barber's Pole" approach, dropping the left elbow and spiraling fingers and wrist around the neck with lower wrist. The index finger points back towards E more than usual but can operate more powerfully than with wrist pronated.
When shifting on the neck everything moves together, with good finger shapes and keeping the same relationship between thumb and fingers. In this way the left hand functions become very mechanical and as simple as possible. Not only the thumb but also the index finger keep close contact with the neck and stings, between them measuring distances moved as well as position-to-position.