I have a "through the Looking Glasses" idea of making another version of Palm Arduino.

Since this design derived from Palm Arduino, I will called it Palm Arduino II.

This Palm Arduino, the Second, will be the combination of Palm Arduino and Portable 5V Voltage Regulator.
So that everything is staying together closely in the same package!

Palm Arduino II will be a little smaller, 1-3/8" wide x 2-3/8" x 1/2" thick (Palm Arduino the First are 1-1/2" wide x 2-1/2" long x 1/2" thick), so it will be fit on the back of the 9V battery holder.

Palm Arduino II will be powered by separate PCB installed on top of 9V battery holder that has both 5V and 3.3V regulator.

PARTS

Palm Arduino II
Parts for Palm Arduino II are exactly the same as Palm Arduino, less 3mm Green LED and 1K Resistor.
I changed the nuts and screws size to 4-40 instead of 2-56.

1 no. ATMega328P with Arduino Bootloader
~~1 no. 3mm Green LED~~
~~1 no. 1K Resistor~~
1 no. 10K Resistor
1 no. 16Khz Resonator
2 nos. 0.1uF Capacitor
1 no. 1x6 Male Header
1 no. 1x14 Female Receptacle
1 no. 1x4 Female Receptacle
1 no. 1x6 Female Receptacle
1 no. 1.375" wide x 2.375" long, 1/8" thick clear Acrylic sheet
Plastic Standoffs
Some hookup wires

2 nos. 4-40 Flat-Head Machine Screws
4 nos. 4-40 Steel Machine Hex Nuts

Voltage Regulators PCB
1 no.   Slide switch
1 no.   1N4001 Diode
2 nos. 100uF Electrolytic capacitor
1 no.   10uF Electrolytic capacitor
1 no.   0.1uF Capacitor
1 no.   78L05 Voltage Regulator, TO-92 package
1 no.   78L33 Voltage Regulator, TO-92 package
1 no.   PCB about 7/8" wide x 1" long

2-pin Female socket with hookup wire

Step 1: Build Palm Arduino II

Since the Palm Arduino, the Second is the decedent of Palm Arduino Kit.
There is no different in how to built steps between the First and this one, except Palm Arduino, the Second does not have pin D13 LED and resistor.

So instead of repeating the steps, I would referred to these Steps from Palm Arduino KIt:

Step 2: Straighten the pins

Step 3: Added Power and Ground

Step 4: Bring out the pins

Step 2: 5V and 3.3V Regulator PCB

Use the same Schematic of the Portable 5V Regulator, with addition of 3.3V Regulator (See Schematic for details). Which perfectly fit on the top of 9V battery holder (7/8" wide x 1-1/8" long.)

5V Portable Regulator
I placed the circuit right on the 9V Battery holder with double sided foam tape.
The circuit was one side circuit.

I placed the components on the solder side of PCB board, because I wanted to use the component side of the PCB to stick to the foam tape and place it right on the top of the 9V battery holder.

The 5V Portable Voltage regulator consists of

Slide switch
1N4001 Diode
100uF Electrolytic capacitor
10uF Electrolytic capacitor
78L05 Voltage Regulator with TO-92 package
and 2-pin Female socket with hookup wire
PCB about 5/8" wide x 1" long.

The schematic of this 5V Voltage regulator is from my notebook when I started to learn embedded electronics. As when I first learned why the diode was added. Because it blocks current from flowing in the opposite direction. And it will block current from flowing backward and damaging the system if you accidentally hook up the power supply the wrong direction.

To built this Portable 5V Regulator is quite easy.
Just solder the components according to the schematics.
And be careful about the polarity of the the diode, and capacitors.
For the 78L05 IC also need to be careful on the pins connectivity.

I used 2-pin female socket as the power connector.

Step 3: Add the Cover

Cut a piece of Acrylic (I used 1/8" thick) to size (1-3/8" x 2-3/8").

Drill two holes. By using an existing hole on the bottom of the 9V battery holder as the guide to drill the first hole.

Drill the second hole on the 9V battery holder make sure that the hole is not too close to the FTDI male connector. Then use this hole as a guide to drill the hole on the Acrylic cover.

Insert 4-40 flat head screws into the holes from the battery side of the 9V battery holder.

Place the Acrylic cover in place. Put the hex nuts in.

Step 4: Test

Hook up the FTDI Cable
Load the Sample Blink.ino to Palm Arduino II.
Disconnected the FTDI cable.

Since this Palm Arduino does not included the test LED on pin 13.
So we have to insert the LED into pin 13 (LED positive lead) and Ground (LED negative lead).

Turn the ON/OFF switch on the Regulator PCB on.
After a few seconds the LED will blink.

We have the working Palm Arduino II! Yippee!

Step 5: Usage Demonstration: Motors Driver Board (SN754401 IC)

This step was added to show how I used Palm Arduino II to control motors by added DIY motors driver board using SN754401 (or L293D) IC, instead of just showing Palm Arduino II blinking the LED!

Build the Motors Driver Board

Cut PCB to size 7/8" wide x 1" long

Test Motors

To test the motors I used the code from this instructables "Control your motors with L293D and Arduino", shown in step 3.
Thanks guibot.

Overall dimension (included Regulators PCB and Motors Controller PCB, and Jumper Wire Connectors)
1-7/8" Wide x 3" Long x 1-3/8" Thick

Video