1. Ten rare earth magnets, 10mm diameter by 3mm thick. These have a pull of approximately 2kg per pair. I notice that Carbatec sell them, but they're much cheaper off eBay - I bought 50 of them on eBay for the same price as 10 from Carbatec.
2. Some 1mm veneer (Silky Oak in my case, as I already had it). You could use 0.6mm veneer instead - that just means that you'd need to laminate five layers instead of three to get a 3mm total thickness.
3. Some 75x12 timber from which to cut the curved top of the armrest. I re-sawed a piece of 75x25 in half to get the thickness I wanted.
4. Some 0.8mm cork sheeting. Also bought off eBay.
Construction
Note that the pictures show a left-handed armrest, as I play left-handed. Adjust accordingly for right-handed.
1. Cut three pieces of veneer, two long-grain and one cross-grain, to make a three-ply. I cut them to 220x55. You need to decide where to position them on the lower bout. For my purposes (playing classical guitar in the normal classical position), I decided that I wanted the centre of the rest to be slightly below the centreline of the bridge. When I take a line through the centre of the bridge out to the completed armrest, there is 110mm of rest below that line and 80mm above.
Glue the veneers around the lower bout of the guitar, using a couple of bridge blanks and some old bicycle inner tubes as clamps. You also need three small pieces of veneer as packing under the other side of each bridge blank to keep it square to the side of the guitar. I found I needed glad-wrap not only underneath the layers of veneer but also on top, as the glue migrated right through both the top and bottom veneers. To shorten the inner tubes to get sufficient tension for the clamping, I just tied a knot in them.
7. Taper and curve the outside of the top of the rest so that it slopes down towards the outside of the guitar; curve and clean the ends. To get the taper, I cleaned the excess glue off the top, scribed a line about 6mm down from the top, and then removed wood freehand on a sanding belt over the outer 2/3 of the top of the rest.
8. Fine sand and apply a finish. I used an oil finish as providing a surface that my arm would easily slide over.
For the next one I make, I'll taper the rest part of the armrest more - this one looks too blocky. I also think that five magnets is maybe overkill. Three would probably be enough, and with a larger size (say 15mm diameter x 3mm thick, which have a pull over 50% greater than the 10mm I used), two might be enough.
