We now have a product - a basic 4G cell phone signal booster that works on band 20 (800 MHz). As far as a business plan is concerned, this could be sold as a kit where the user does all the easy, labour intensive soldering and simply buys the main PCB ready populated with the fiddly SMT components. It can also be hacked to work as a general purpose microwave RF digitally controlled amplifier for experimentation, but be warned - this gadget is capable of quite high transmission power - up to 1/2 watt - and a special operation license may be required. As a cell phone booster it works at a power in the order of 1 mW with a maximum range of about 10 metres so does not risk interfering with the neighbours too much.

Here, above, the massive conglomeration of wires and circuit boards of the prototype on the left is miraculously shrunk into one neat unit with only two loose wires which will connect to the indoors and outside antennae. Fear not, the device is still fairly hackable and even supports addition of the nice pretty Adafruit TFT screen with special mounting holes and connections, if required (but not essential).

Finished, fully assembled, Arduino Cell Phone 4G Signal Booster as below. I did a quick test today and it actually worked! Still some room for improvement though.Schematic is HERE.

PCB design file is HERE.

BOM is HERE.