Initially, I tested the connections using M/F jumper cables and an ItsyBitsy M4 with male header pins soldered on (the final build required a fresh ItsyBitsy due to the different header pin orientation used).

This was good for proving out the pin assignments, but not entirely as ergonomic a package as desired.

To build the proper adapter board, we'll use the ProtoBonnet. To it, solder the 2x20 male header pins and the two 14 pin, single-row headers spaced for the ItsyBitsy as shown here.

A good way to align the single-row pins is to tape the ItsyBitsy to the board as shown, then flip the board over for soldering.

Wait until the end to solder the ItsyBitsy itself onto the male headers, so that you can have room to solder the wiring in place.

Carefully follow the wiring diagrams and begin soldering short lengths of wire to make the connections between the Pi pin breakout row on the ProtoBonnet and the associated pins of the ItsyBitsy.

I found it easier to tack-solder some of the wires from the bottom side before flipping over to fully solder.

You'll notice there is one Pi pin we need to use that isn't broken out to the ProtoBonnet -- GPIO 26, which runs to D2 on the ItsyBitsy. For this one, we'll solder the wire directly to the header pin as shown.

Now you can place the ItsyBitsy M4/M0 back onto the header pins and solder them all in place.

While wearing eye protection, use diagonal flush cutters to trim the excess lengths of the ItsyBitsy header pins ONLY! Do not trim the 2x20 Raspberry Pi Bonnet headers!!

Before connecting the ItsyBitsy adapter board to the acrylic base, do a test fit to make sure the board connects properly to the Keybow.

Now, you can connect the ItsyBitsy adapter board to the Keybow base and fully assemble the board using M2.5 nylon standoffs and screws.

Follow the original Keybow instructions for this, substituting the ItsyBitsy adapter board for the Pi Zero. Since the wiring and header pins make this board a bit taller than the Pi Zero you may need an extra bit of room to fit it by adding a longer screws in a couple of spots. The nylon M2.5 screw and standoff kit is great for this.

Pi Headers

Place the 2x20 male Pi header pins into the board as shown.

You can use the Keybow to hold the pins in place as you solder them from the back side of the board.

Itsy Headers

Next, fit the two ItsyBitsy male header pin rows into the board as shown.

You can hold them in place with some poster putty or tape and then flip the board over and solder the pins in from the back. Push the pins flush with the board with the soldering iron as you go for the best fit, or trim the extra points off later with some flush cutters. (The Pi Zero we're replacing has a totally flat back.)

If you want to go the extra mile, do not solder the ItsyBitsy in place yet! We're going to remove the headers' black plastic spacers to get a super low profile so the ItsyBitsy will fit neatly in the case.

Alternately, you can solder the ItsyBitsy to the unmodified pins, but you may need some extra spacers for the acrylic case.

Carefully pry up the black plastic spacers, you'll be left with bare pins (or a very tiny bed of nails depending on your perspective).

Now, solder the ItsyBitsy M4 to the pins, and then trim off the excess.

Mounting

You can now mount the Itsy/Pi adapter to the acrylic case -- it has the same dimensions as the Pi Zero it is replacing.

Affix the Keybow to the board and off we go!