This thread inspired me to improve the DIY mount I had, which was very similar to OP's
There are some disadvantages to strapping it on top of the stem, or a stem bag.

I took a 2" wide strip of aluminum, about 13" long, and made an out-front mount for my phone. A 7 inch device might need a little longer. I can post a picture later if you like, but it was surprisingly easy to make and I'll briefly describe how.

First, using pliers simply bend one end in a curve to fit the top of your device, and attach some rubber there and as far as the length of the phone or tablet. Put the phone on it and mark on the strip at the phone's bottom, and drill a bolt-hole there. Drill two overlapping holes to make it adjustable. Then just install a bolt sticking up, snug against the device. That by itself was enough to hold my phone securely but for good measure I added a double-sided velcro strip around the phone at the bottom.

Pad the close end, slide it on the underside of the stem and strap it on. I drilled 4 holes and used zip-ties, but however you want to affix it ... Gently bending the mount up in front to get a convenient angle and position. Vibration doesn't seem to be an issue.

"Waterproof" inhibits viewing, so I think it's best to stick with the tried and true sandwich bag for wet conditions.