I'm sorry, I have never made a metal resonator CBG. But, I am supporting the Reso CBGs by providing links to other places on CBN where they do talk a lot about Reso-CBG. Look at my list of useful links on the first page of this discussion group. The fourth section lists three other discussion groups whose readers are more likely to have a useful answer than I. You can also try to use the Search feature on CBN, using key words like "install" and "resonator". You can also search for photos, which may then lead you to some discussions. I know that resonator cones from different makers can require different methods of attachment. I understand that Old Lowes cones are easy to attach as they are a bit sturdier and have mounting holes around the outer rim, allowing you to screw them directly to the soundboard, eliminating any need for any cradle arrangement. That's the way I'd go.
Here is a photo of a nice Reso-CBG built by Darren Brown (a CBN member) using one of Old Lowes' brass cones.
Notice how the resonator cone is screwed down to the sound board by 12 screws. This is about as simple as you can get for installing a resonator. The other thing to remember about installing a resonator cone is that the cone holds the bridge, and it's location is important because the nut (or fret 0) and the bridge determine the scale length. On this photo, the resonator is also pushed back toward the tail piece to allow room on the soundboard for picking and strumming the strings. There are also two sound holes (or groups of holes) on either side of the neck thru the box.
If you are using something else for a resonator, then look around for some photos of how they did it. People have used tuna cans, hub caps, license plates, stove top burner cover plates and lots of other things for resonators with mixed results. Many resonators require a "cradle" or "sound well" upon which the resonator rests within the cigar box. Others use a tripod (three point) system of holding the resonator up off the bottom (back side) of the cigar box. The key things to remember are: 1.) The thinner and lighter the resonator material, the better it will resonate (thin aluminum will be better than thin steel), 2.) minimize the number of connection points of the resonator to the box, as connection points can dampen the vibration. There are probably some other valuable advice, but I'm still a novice when it comes to metal resonators. Canjos are about all the experience I have with metal resonators. If I find out some more about mounting resonators I'll add it to this discussion.
I was mistaken about the Reso-CBG being a neck thru design. Most neck thru designs can't be used for Reso-CBGs as the resonator cone wants to go where the neck goes thru the box. So, most Reso-CBGs either have a bolt-on neck, or have a couple of stout cross pieces built into the box so that the neck can be connected securely to the box. Here is a photo of one such box by Kev Lloyd, another CBN member.
Hmmm... Why can't I insert a photo here???
Will try again...
Here's that photo. See how heavy duty his neck bracing is? That's what you'll likely have to do. With a metal resonator, having such heavy internal bracing shouldn't hurt the sound of your box that much, at least according to my way of thinking. I haven't actually built one. Anyone with real experience is welcome to add comments here.
P.S. As I recall, this builder was using a metal doggy water bowl as a resonator. Never heard how it came out.
Keep us informed on what you build (e.g. what you used for the resonator "cone", the biscuit, and how you mounted it in the box). It's an interesting topic, and with all the different things you could use for a resonator, the possibilities are nearly limitless. As I get into this topic more, I'll likely try my hand a building a few. I have actually been putting it off, saying to myself I'll wait till I'm back in the States and buy a few Old Lowe's cones. But, that's a weak excuse. I could try a paint can lid or tuna can build in the mean time. What I have built so far are some wood boxes that I've incorporated into a couple of box guitars, and my current build is a "paddle shape" or "McNally Strumstick" shaped instrument. Once that's done, I should build another box incorporating a resonator of some kind. It will give me something to research while the glue dries. If you check out what I have built, you'll know I find CBG building very fun and intellectually rewarding as well. I even like to play them, or "noodle around on them" (which probably is more descriptive of my playing skills).
For more up-to-date info on my experiences on building sound boxes for use with metal resonators, check out this discussion: Notes on Making Sound Boxes for Metal Resonator CBGs. In this discussion I show how to build a "Reso Box" using a 6" round Eu Long cookie tin lid. Here's a photo...Additionally, Michael Fred Johnson and Tom Walters (both are CBN members) have added discussions on making stringed instruments using cat bowls for resonators (apparantly, the cat bowl is a nicer size for 3-stringers as compared to dog bowls). Here's two links to those discussions: Box for three string reso and My CATBRO Resonator.