To hollow out the two sides I start by roughly removing the majority with a knife. (Photo, top-left). Just slice out big chunks of the soft stuff until you have most of it out of the way. To remove the rest, and to shape the interior, a number of methods can be used. One quick way is to use a wire brush attachment on your drill or drill press and simply 'brush' it out. Remember the outer shell is hard and will hold up well to the brush. This method is quick but tends to 'gouge' out valleys as you work along. I don't like to remove all the interior right down to the shell. I leave a small thickness of it in. Another thing you can use is a rotarty rasp attachment on your drill, used similar to the wire brush. Lately my favorite way to shape/remove the interior is to use a hand rasp with a rounded bottom. This takes the stuff down in nice even amounts. Leaves the interior nicely shaped. I then touch up any rough spots with some course sandpaper.
Construction of an Agave Didjeridu