I have retro-fitted the VXQ1911 part in the back of my Panasonic 7-14mm. It cost me a blummin' fortune from www.partsimple.com... USD 79.24 including shipping to Thailand. It was the cheapest option I could find. I used a craft knife rather than a Dremmel to shave off the material that wasn't needed.
Using tape does work OK. Use the thinnest 3M Scotch Magic tape which leaves less residue than others.
As for filters, in my experience the Original Magic Filter does not give good results underwater with the GH4. I don't know how their Auto-Magic filter performs. In the back of my Panasonic 8mm fisheye I have recently been using a piece of Rosco Dark Salmon (#008) e-color+ gel which perfectly matches the UR-Pro blue-water filters that are used in Gates, L&M etc. housings. With a manual white balance on the palm of my hand, results seem good. You can contact Rosco or their distributors and ask them to send you a set of Rosco swatch books for free then you can test different colours. The swatches themselves are big enough to cut a couple of filters out of. Be careful not to scratch them as you flip through the swatch book. These gels are intended for lights, not lenses, and purists will tell you that you should be using a proper optical gel filter such as Kodak Wratten, but the Rosco material looks pretty clear to me! I don't know which of the Rosco gels is theoretically the most optically clear. Once I know that I will probably order a bigger sheet. I have also used Rosco lighting gels as ND filters on the back of my Panasonic 7-14mm.