Found this case on Amazon for hard drives. The dimensions are 6.0" x 3.7" x 1.2”. It's slightly larger for the length of the 351v (live very slightly), but it's not bad and it doesn't move around when its zipped up.
This is a great one too for $10, shared by other folks around here : https://smile.amazon.com/dp/B006P2R708
There are many variations of brands/colors too in the suggested items, all listed as Hard Drive cases. Personally I went with red since it sticks out when its tossed in a bag.