The sound of an acoustic resonator guitar is unmistakable--crisp and penetrating. The original resonators were created with hand tools 90 years ago, and you can similarly create your own resonator with a few tools and some sweat equity. Be warned, though: Many people have considered the project and decided it's cheaper, easier and just as good-sounding to buy an inexpensive resonator. If you decide to go the DIY route, though, here's how to get it done.
Decide whether you want a Hawaiian (unfretted) or roundneck (fretted) resonator. Basically, this means deciding whether you want to play the guitar with a slide or fret it with your fingers. You can raise the action on a roundneck and play it with a slide, but you can't lower the action on a Hawaiian to fret it. This decision will help dictate the guitar you buy.
Pick a guitar with laminated top and sides, regardless of the style you're going to play. Laminated tops and sides are stronger, and since they're not responsible for producing the sound, their lack of resonance is a non-issue.
Choose a comfortable neck if you're playing roundneck. Find a neck that fits your hand comfortably. Also, feel the fret ends. Do they poke your hand as you run it up the fretboard? If they do, choose a different guitar.
Buy a high-quality cone. The cone--the series of metal baffles and plates that give a resonator its distinctive sound--is the most important part of a resonator. A cheap cone produces a tinny, rattly sound. Quarterman cones are regarded as the best; they're available from Janet Davis Music (bluegrasscenter.com).
Buy the accessories that go along with the cone. You'll need a soundwell--a wooden ring with holes that supports the cone--as well as a cover plate and spider bridge to go over the top, a bridge insert for the strings to pass over, wood kerfing, plastic binding, and screws to hold it all together. Beard Guitars (beardguitars.com) has these, as well as a blueprint that'll be vital as you begin building.
Choose a substantial tailpiece for the guitar. If you think you'll be playing alternate tunings, choose a Trilogy tailpiece from Hipshot (hipshot.com). The cost of all these will likely exceed the price of the guitar to be converted, but that's normal.
Use the cone as a guide to outline the hole to be cut, then cut it using a jigsaw or coping saw. Smooth the cut but don't be a fanatic. It just has to fit; it doesn't have to fit perfectly. You'll cut through braces; that's OK. The tailpiece and spider bridge will bear the string tension. Cut two 2-1/4-inch holes in the upper bouts as soundports. Cover them with screens.
Insert the soundwell, then place the cone on top of it. Cover it with the cover plate and spider. Refer to the blueprint for the placement of kerfing and binding. Attach the tailpiece to the back strap pin. Place the bridge insert into the bridge.
Remove the old nut and install a Dobro nut if you're playing Hawaiian. Otherwise, string it up to a relatively low-tension tuning like an open C and see how it sounds.