For some reason, a recent thread on this topic by @Lenny Nichols was closed without anyone posting a photograph of how this can be done. I used an H-Clamp. Quick, easy solution if you need this type of solution.
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Interesting, what did you add to actually hold the phone? How much weight do you figure is reasonable? And, do H-clamps have any impact on movement of the top plate?
Call me old fashioned but that just looks terrible. (to me.)
I wasn't trying to harsh you. I think H clamps look bad, too, "by themselves." But, at least with an RE-20 attached, their purpose is obvious. But why do you need to attach a phone to your bass?! A simple, standalone mic stand w/ a gooseneck with appropriate hardware would look fine, like the ubiquitous iPads on stages everywhere these days.
Heck, even one of those Bob Dylan harmonica racks around your neck would look better.
Especially if you don't care about it affecting the acoustic sound.So if you're interested in a use case such as enhancing ordinary performance video, the benefits of of mounting a smartphone on the double bass may well outweigh the downside of some people not liking how it looks.
I have a friend that uses velcro to attach his phone to the upper bout and uses it on gigs for charts. Every time one of his buddies catches this at a show, they call his phone and watch the chaos when it drops the chart!
Back to the OP, what's the goal in this? Mount a smartphone to act as camera/ recorder / live-stream?
Mount the smart phone to read chord charts?
For both, I think using a clip attachment on a mic stand is better. Video will be bump / vibration free and with a wider angle. Charts will also be in a better position to read, change pages and will also encourage better player posture.