I've made a bunch of preliminary decisions:

1. esp8266 + lipo charger + usb/serial converter as the 'brain'
2. 4 way bus: i2c, power and ground to connect all modules
3. attiny88 for the modules (cheap and supports hardware i2c slave)
4. first module will be double push button encoders with led dials
5. second module will be an lcd 2x16
6. the sides of the box will be 20mm rexroth type aluminium profile, front and back will be laser cut & engraved acrylic, end plates will be wood, delrin, aluminium or acrylic.
7. boards will be joined with a connector like this http://es.farnell.com/avx-interconnect/00-9159-004-061-916/card-edge-connector-open-ended/dp/2347864?CMP=i-bf9f-00001000

I want to use an lcd like this http://www.farnell.com/datasheets/2021823.pdf?_ga=1.177301349.1169383904.1463992672

size is 85.0 x 32.6mm

or perhaps an oled.

I'd also like to have a moving coil type dial as a possibility:

http://uk.farnell.com/multicomp/sd506-0-15v/battery-charge-meter-0-15v/dp/4434079

which are 44mm square.