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The hinges were sunk into the wood so that nothing would catch when the cajon was moved or slid on its side. It's also nice to take into account the sensual aspects of touching the instrument. Supports were put in place to prevent the chiseling from fracturing any of the recently-glued joints. The hinge that meets the door lays on the inside. This creates a small space between the door and the frame against which it will lie. All along the frame for the door was glued a strip of used bicycle inner-tube rubber, which was, conveniently, just about the same thickness as the hinge, the idea being that a good seal is important and a material with some give and sealing properties was needed to achieve that. It may not be quite enough: two layers of rubber may have been better: one glued to the door, one to the frame, and the clasps adjusted to appropriate tension. Otherwise another, thicker material may be used, but as it stands it seems to work well enough.
The door was a lot of work, but for an experimental cajon, it seems worth it. Of course a cruder, less-easy-to-use door could have been fashioned, although screwing into the edge of plywood is generally not realistic. The clasps, though they may appear to be casually located, are actually very carefully placed so that when the door is closed and the clasps engaged, the door is under tension against the frame. It is nonetheless possible to notice the door moving slightly when the kick is hit, which indicates that it is absorbing some of the sound despite the clasps. The shape of the clasp is such that maximum tension is reached before the clasp is fully closed, at which point the tension backs off. This keeps clasps closed when used normally, e.g. to keep a cabinet closed. In this case, they could be bent out, so that maximum tension is sustained when they are fully closed. Then they could be placed and adjusted appropriately and a better seal attained. Additionally, one could place more clasps along the top and bottom of the door frame. Make sure you give the top and bottom clasps enough distance from the framing pieces so that they can be engaged and disengaged.
The edge of the door that meets the hinges needs to be shaped, of course, in order to allow it to open (in this picture, the door is the piece on the left, and the visible hinge is attached to the side wall). For fine-tuning the snares, it is recommended that the door be able to open wide enough to allow fiddling with the snares against the tapa while the cajon is tilted back at a playing angle, since the tilt can significantly affect the force with which the snares meet the tapa.
I couldn't find large, tough, rubber feet with holes, so i used rubber stoppers with a screw and a large washer. I drilled a small hole for the screw, then drilled with a larger bit halfway into the stopper, sorta forced the screw and washer in there, and voila. Drilling into rubber is difficult and probably dangerous. Perhaps freezing the rubber first would help. Can't honestly remember if I tried that. :-) I think I did, and it was useless. Anyway: generally, the feet in the back are large to enable the player to tilt back. The feet in front are less important. When designing your feet, watch where you're drilling and plan ahead for the screws that will be coming through the front tapa and through the door frame in the back. I intentionally mounted the screws for the feet into the hickory frame inside, for added strength.