It sounds like retroplayer was starting from the enclosure side, and I was starting from hooking up 20 1/8" (3.5mm) phono adapters. However, as I read it again, you don't really need 20 inputs with 1/8" plugs, each of which contains two wires for a digital sensor, or three wires for an analog sensor.
One thing you can do to organize the wiring it to ethernet cables to connect multiple signals together. Ethernet cables contain 8 wires, so typically you would be able to attach 6 sensors, a ground, and a power wire to each cable. Adafruit sells a nice little patch shield that fits on an Uno (or other microprocessors with the same shield layout) that provides 4 plugs to plug in ethernet cables, and then 4 connectors on the other end to distribute the wires to a breadboard or protoboard: https://www.adafruit.com/products/256
They also sell panel mount ethernet mounts if you want to spiff things up: https://www.adafruit.com/products/909
. You would mount these on panels, and have the inner cable connect to the patch shield.
If you ever wanted to take your device outside, Adafruit also sells ethernet cable glands that allows you to put your Arduino & battery into a waterproof enclosure, and then connect to each of the sensor platforms: https://www.adafruit.com/products/827
A different way to wire things up if you don't want to go down the enclosure route, is getting one of the various sensor shields out there. These shields break out each Arduino Uno pin into a set of 3 pins (ground, data, power), that you can then connect each sensor easily. Note, some sensor shields have a different ordering for the digital pins and the analog pins, so be sure you know how yours is wired up. Here is one for the Uno: http://yourduino.com/sunshop2/index.php?l=product_detail&p=2
and one for the Mega: http://yourduino.com/sunshop2/index.php?l=product_detail&p=195