I've not heard of any general issue with Humminbird product quality in their low-end products, quite the opposite. I have an older 400TX fish finder that has worked fine for years. The quick detachable head is a definite plus. I've just bought a side scan transducer for it as well as a used earlier 3D head (nice that many of the early Humminbirds share the same mount) and will be trying those out.
For mounting I used a short length of 1/4" aluminum angle (3" x 3" legs) to fabricate a bracket that bolts to the transom and extends out over the tube top. This involved simply cutting one leg of the angle to match the tube curvature so the bracket can extend out over the top of the tube (but leaving a nub of the leg along the extension for stiffness), and drilling a few holes for bolting to the transom and mounting the sounder base. I set the bracket off from the transom face with some stainless fender washers and positioned the bracket a little above the transom top - this was so the bracket wouldn't touch the fabric attachment points on the transom face or top. It's easy to cut and finish the aluminum with a sawzall and hand files or drill/dremel sanding drums.
I put a similar bracket on the other side for the downrigger mount base, and made both brackets long enough for a couple rod holders on each side. It works great and is more useful and reliable than when the finder was in a pelican case with lantern batteries. In the future one of the rod holder bases will double as a mounting point for a detachable stern light for night use, and I'll shortly be trying the downrigger mount base with an electric crab pot puller.
These brackets have proven to be a nice feature, and didn't take long to make at all. I used 1/4" thick brackets for rigidity to mount rod holders and the downrigger with cantilevering - you could go with 3/16 or maybe even 1/8 for just a head unit mounting point, for even easier fabrication, and the cantilevering over the tubes is a matter of choice, provided whatever you mount on the bracket doesn't interfere with the motor's range of motion. The next project is a more refined deployment method for the transducer so I can get it down in the water stream for planing use.
Good luck!