I finally received all the needed parts to build my own LCD/SD panel for Felix.
Since I wanted to integrate the LCD/SD panel as neatly as possible I ended up designing a quite unusual solution

The lower part of the LCD panel is actually inserted into the slot in the aluminum extrusion (with printed protection in front and back). No screws are used and the panel is press fit into the extrusion. (This led to a little problem with the current printed version; I was two layers off for the width of the slot in the extrusion and had to remove two layers manually with a knife from the printed parts

I was able to assemble everything tonight with only a few teething problems that are easily fixed.
A little video of the panel.
Everything is working at the moment except the SD card reader. At least it did not blow up the card I'm using (an ancient MMC card that pre dates the SD specification!). I'll try with some of the other SD cards I have laying around. I might have to buy a level shifter or just buy a SD card reader with it integrated, but that means a (minor?) redesign.
I choose to omit the "emergency switch/RAMPS reset switch" from the design since that is available on the front of the electronics box. I didn't include any buzzer either - didn't see the need for it and it can be easily added inside the electronics box if needed/wanted at a later stage.
I have hidden two 10K potmeters inside the panel to control the contrast and brightness of the LCD panel (only brightness pot/resistor is really needed). While reading up on the many variations of the 20x4 LCD displays I got worried that I didn't have the correct parts ohm-values needed to adjust the voltage of the backpanel correctly. But it seems that the model I bought had the needed resistor to lover the voltage down from 5 volt by it self and luckily the 10K potmeter worked nicely. Drama avoided...
The wiring of the rotary encoder was also different than I had expected and I had to try several variations of the wiring before I got it working. Another big problem was that I started the build folowwing the excellent PanelMax tutorial. The only problem was that it was outdated compared to the current Marlin version used by Guillaume and one of the pins was already dedicated to Fan out, creating "interesting" results with the menu alternating constantly between the main menu and the first sub menu. I changed the Fan pin number but later remembered that it would have been better to use Aux 4 instead for the LCD panel and rotary encoder...
I only had 20cm dupont wires available so had to solder a wire extender adapter for the LCD while testing.
The SDCARDDETECT Hack described in the PanelMax tutorial is not needed - use Refresh command in the LCD menu (this is missing from the Marlin version I have!).
I plan to leave AUX1 free for possible future Bluetooth upgrade.
I will upgrade the wires later for neater appearance. Unless I just en up completely redesigning and integrate the LCD and SD card reader into the electronics box

Parts used:
SD card reader: SD Card Module Slot Socket Reader for Arduino ARM MCU - not proven to work reliably yet
Info about the SD card reader being compatible: LCD Upgrade
20x4 LCD Display: HD44780 2004 20x4 White Characters LCD Display Module + Blue Blacklight - works nicely
Rotary encoder: EC11 car ROTARY ENCODERS 20 Position Step-level Modulator for DVD CD Speaker - works nicely
Momentary push button: Red LED 12V Lamp Round Cap Self-locking Push Button Switch 3A 250V AC - I was a little uncertain about this one and omitted it for the current design
In addition I bought some other bits and pieces:
2 x 10K OHM CERMET POTENTIOMETER - 1 used for LCD backlight dimming. The other is usable contrast on LCD panel or for dimming the push button LED
25 x 1N4004 Diode 1A 400V - D1 on RAMPS - to be able to print from SD without USB cable attached - not installed at the moment
10 x 40Pin 2.54 Single Row Pin Male Header - cut to size and soldered on the RAMPS AUX ports.
80 x Dupont wire cable 1p-1p female pin connector 2.54mm 20cm - bought due to the many different colors and pre-crimped terminals - too short should have gotten 40cm or 60cm

25 x 10P Dupont Jumper Wire Cable Housing Female Pin Connector 2.54mm Pitch - will be cut down to correct sizes
10 x 5x7 PCB Prototype board - only one "needed" but the price difference was minor
I'll try to write a tutorial suited for Felix users so that you do not have to go trough all the trouble I had to should you want to build a LCD/SD panel yourself.
I'll sort out some kinks in the current design before I release the files. Let me know if you are interested and eager to try it out
