Good timing! I was just about to go out to my storage area for something, so I snapped some photos of my mount for you. I have one built from a length of 2x4 and some lag screws. It's not pretty, but it works well. I built it in 2016.
I bought lag screws with heads the same width as the Miata's Frankenstein bolts (you can test the bolt heads with the hard top safely on the ground). I put them into the 2x4 at the appropriate distance, after pre-drilling the holes to prevent splitting. I left them sticking up just enough to be able to slide the hard top bracket onto the bolt head. To attach this apparatus to the wall, I used three longer lag screws with washers, spaced to go into studs. Predrilled the holes, of course. Last, I got an eye screw with a bit of rope to serve as a safety catch in case someone or something were to knock the hard top loose and make it fall.
It has never fallen, so the rope is probably overkill. People may say this isn't a good way to store a hard top. I can only speak for mine, but it has been stored this way for six summer seasons with no harm to the top whatsoever. I bought a simple dust cover for it on Amazon and slit holes in the seam for the brackets to stick through.
Photo of bolt head with measuring tape - looks like I went with 1/2-inch.
Hope that helps you!
