Audio Technica lavaliere condenser microphones like the PRO 70 and AT831B are good choices for amplifying acoustic guitars and reproducing the tone of the instrument more accurately than pickups. The soundhole mount that comes with is also quite nice, but after a few months of use I lost the screw and clip attachment on a gig. This made the microphone essentially unusable. I was quite annoyed to find that a replacement mount would cost around 30 bucks!
Subtle changes to where and how you mount them can make a huge difference in terms of sound quality and feedback suppression. So I designed two styles of mount that accommodate some of the different approaches I've encountered.
The first mount slides between the strings behind the bridge and the microphone can be in turn pointed straight down at the top of the guitar. This is for guitars that feedback almost instantaneously from the sound hole, which is often the case with large soundholes like the "D-Hole" style of gypsy jazz guitar.
Overall this location results in a brighter, more treble-y sound. Both the distance from the bridge and the mic distance from the top can make some rather dramatic differences to the tone, though. Moving to the mount to the lower strings also brings out better bass response.
I like this design because it's so small that you can just leave it on your guitar. When it's time to amplify you can just slide the mic right in and be ready to go.
Soundhole Mount (Interior option)
I noticed that many players like mount inside the sound hole. This can really help fight feedback and interference from external noise. There are a few issues with this approach: it can be extremely hard to actually get the mount and microphone in the soundhole without loosening the strings or using some severe finger contortion. Also there's the issue of the dangling cable. I've seen players tape it down to the top with duct tape! I addressed this by making the mount thinner, and including an extra clip on the other end for the cable.
The thin profile makes it easy to slid under the strings. Once it's there I reach from the other side and slide it into place. The cable clip keeps the cord away from the strings and sends it out of the way.
If you flip the mount around, it can be still used as a exterior soundhole mount. I adjust the height by sliding the microphone and/or mount up and down. I haven't found the need for much more than the 20mm max height of the print, thought I did make a 40mm version just in case.
There are two sizes to the soundhole mount. One has a 3mm top clearance and one has 5mm (for thicker guitars, like archtops). You’ll probably need to add your own felt to the latter one if you want to protect your finish. I just glued some in there with superglue.
The bridge clip can be printed without supports if you point the clip portion straight up