This instructable is for two variations of a cajon kick. Honestly, if you are making one, you might as well make two. It's super fun to go heel toe on them and have a kick and snare.

I designed this to slide underneath the cajon, so I can clamp it by sitting on the cajon. It works real well. It also means that you can slide it underneath other things, like suitcases, gas canisters, cardboard boxes, etc. So you don't exactly need a cajon.
The other bonus is that I don't have to mod my cajon to fit it on and I can easily adjust where it hits or what side it hits by simply sliding it about.
Super simple build for an afternoon.

Supplies

Per pedal:
1X Bolt
2X Washers
3X Eye hooks
2X Springs
1X Hinge
Screws for the hinge
2X Hose clamp (optional)
1X felt or material (optional)
1X drumstick, brush, or kick beater (optional)

12-18mm Plywood or other decent wood.

Step 1: Designing the Idea

Most of the time when I'm designing stuff, I do a few quick sketches on Inkscape, but this time around, I thought I would try Fusion 360 for a quick mock up. The Fusion 360 design was based on the idea of running a bike brake cable to another pedal. I built it, but I had a lot of issues with the cable slipping and in the end, I decided that I could just mount the pedal straight to the beater. I also originally intended to use a tennis ball for the beater, and I did that, but cheap tennis balls are really solid and it wasn't the right tone.

So after trying to make this and not being happy, I changed my prototype and made a very simple idea.

I find with a project like this, Fusion is pretty good to get a quick visual on the idea and how it might work and it's easy to make stuff look pretty. If I were really clued up, I could have built the whole mechanism and tested it with animations and all sorts, but once I have my basic idea, I just enjoy manual prototyping much more.

Step 2: Cutting Out the Parts

Each pedal has five wooden parts to cut out:

2X sides
1X beater, or stick holder(depending on your idea, see final step)
1X pedal
1X baseplate

Important things to remember:
The sides should be the same, just mirror images. They can be just triangles, you want them to taper to the top, so that you have room for your foot to move about, but they are sturdy at the bottom when the kick hits the drum.
The baseplate should be as thick a piece of wood as the feet on your cajon. Measure from the floor to the underside of the cajon. Since I built my cajon, it's feet are just plywood rings. So I used the same plywood. If your Cajon sits quite high off the ground, you could add a bit of foam to give it a bit better grip.

Also, see the final step for a different way to do the beater.

Sizes?
The sides are about 170mm high. Anything between 200 and 120mm would work, but it will change the feel of the pedal. The higher the sides, the more adjustment you can build into the pedal height. 90mm at the base tapering to 30mm at the top.

The beater is about 150 mm long. This hits nicely in the center of the Cajon I have, but you might have to adjust to yours. Think about leverage too. The longer the beater, the harder it will hit the drum and the heaver the pedal will feel.

The pedal is about 220mm long. I made two pedals and I personally prefer the longer one, because it's got more leverage and is more sensitive. Shorter ones are a bit more like an on off switch. They work well when you just want a steady driving back beat. Longer is nicer for a snare, because you can move the foot back and get a lighter hit or forward for a more solid hit.

Step 3: Assembly

Step 1 is to screw the hinge to the pedal. It's important to do this because you want to make sure that the pedal clears everything when it swings, so it's important to sort this out.

Step 2 is to drill the holes for the bolts to go through the sides and the beater.

Step 3 is to put the bolt through, and figure out where to attach the sides, so that the pedal clears them properly.

Step 4 glue and screw them on.

Step 5 is to drill a small hole in the back of the beater for a rope to attach, put the end of the rope inside and screw a screw in there to hold it in place.

Step 6 is to drill two holes in the pedal. Why two? Well, it means that you can easily feed the rope down one and up the other, adjusting the pedal height by tying a couple of half hitches from the easily accessible top side.

Step 7 Add some eye screws for the springs to attach to. You could also use elastic for this, basically you want a good amount of tension from both sides pulling the beater back away from the drum.

Step 8 Wrap the beater in felt or some material padding

Step 4: Testing and Re-iterating

So, you've finished it now, but... not everything will be as planned. You might have to adjust the height of the string for the feel you like on the pedal. I found that my springs were slightly too tight and by over stretching them, I could get just a little more slack and get the feel that I liked.

Also, I redesigned the beater. I made a beater with two hose clamps attached to it instead of a beater. One for a vertical drumstick to attach and one for a horizontal drumstick to attach. This changes the utility of the pedal because I can attach brushes, I can attach shakers, I can attach drum sticks, rattles, etc. I can also sit a snare drum on the floor and have the pedal beating vertically down onto it.