One of the problems with the soft interior that occurs inside the agave is that it is acoustically retarded. To leave this soft interior alone will make the didj sound muffled. So you must coat this interior with something to make it a little more 'reflective' to the sound waves. This can be done with wood glue, which I have used and it works 'ok'. The problem I've noticed with the wood glue is that it tends to break down with the contact of your saliva. My prefered method is now to use a two part epoxy resin to coat the whole interior of the two sides.
Before coating the interior you should plug any large holes that might 'leak' when the coating is added. There is a grub that just loves these agave plants and they tend to eat holes through the sides of the stalk from time to time. Personally I use a good wood dough to plug these holes. It dries hard, doesn't tend to shrink, and can be sanded smooth like the natural wood.
I like to use a sponge brush as shown in these two pictures above (black sponge, wooden handle) to spread the coating. The brushes are inexpensive so they can be discarded after use. I mix the resin (two parts) into a paper cup and then just pour it along the length of each half and spread it thin with the brush. Make sure you prop up the two halves so the resin doesn't spill too much.
Construction of an Agave Didjeridu