That’s perfect I was about to do the same. I was able to hang Navio off the top of the APM anti-vibe plate but this one is much better.
Some variants I’m considering building are with the top of this plate merged to the bottom of a 3D printed Navio case to get a direct case to frame mount. Or the same with detachable (screw/cable-tie) points.
About the solid thing that is an artefact of 3D editing software. Something which can be introduced during grouping and export. Depending on which software you used to make it, there are ways to fix easily, but it’s much easier to do with the original 3D design files than patching the exported STL.
For example in Sketch-Up the fix is to install the solid tools analyzer and use it’s wizard to identify the weak links, e.g. lines which are fractions too short. The other way is to click on each face and check that there are minimal cross-sections/triangles, then erase them to force the 3D designer to re-calculate as one solid face (or totally destroy it so you know you have to re-create because it was too far off straight). Of course 3D objects are rendered with triangles but the object in the editor should have as few points as possible.
When it is not solid usually one of the additional points/lines is a fraction too high/low or not properly “grouped”, so the 3D slicer software thinks it’s a case with a crack in the middle, not a solid.
After much pain with “free” tools like SketchUp (which turns out not to be free in the end because solid tools are not included for free = useless for good 3D print design) I switched to TinkerCAD (or you can even use AutoDesk’s bigger Fusion 360 variant as a hobbyist). Not only does that work much more simply with easy graphics and UI like SketchUp (in my opinion better) but it directly pushes the user towards working with 3D printer friendly solids and has a unique idea about simply changing the material of objects/groups from a colour to “hole” in end effect making additions and subtractions (equivalent to “professional” solid tools in SketchUp) and really works for free!