If you have access to a CNC mill and you'd like to make your own custom PCB, you can download my button PCB eagle cad file. I was able to machine this PCB onto a FR-1 single-sized copper board using a 1/32" flat endmill on the Bantam Tools desktop CNC.
Next up we'll secure 4x buttons to a mounting plate and wire them up. I CNC milled a PCB to make this easier, but you can use a 3D Printed "PCB" to do similar. You'll need five new pieces of wire to connect the buttons to the Pi. These wires can be 30AWG size and about 16cm in length.
You'll want to use wire strippers to remove a bit of insulation and tin the tips with a bit of solder. I used different colored wires to help tell apart the various button connections. Some pieces of heat shrink tubing a great for keeping them wires bundled together.
Seat the buttons onto the PCB (the 3D printed mounting plate) by inserting the legs of the buttons into the little holes and press them down to fully seat the buttons flush with the PCB. Try to avoid bending any of the legs on the buttons.
Connect Wires to Buttons
Now we can connect the wires to the buttons. Each button has four legs – two for signal and two for ground. You'll have to tie all of the grounds together so that all of the buttons connect to ground. Then, connect the rest of the wires to the signal leads, which are on the other side of the ground connections. This might get a little hairy, hence why I CNC milled myself a PCB =]
Connect Button Assembly
And with our buttons wired up, we can now connect them to the Pi. We'll solder into the available GPIO pins on top of the Speaker Bonnet.
Connect the wires from the buttons to the following GPIO labeled pins on top of the Adafruit Speaker Bonnet. Note, these are not the actual pin numbers, these are the GPIO numbers, which are totally different. They're conveniently labeled on the Speaker Bonnet.