I'm using Legos a lot to build quick prototypes (e.g. my robots). Now that I have an Arduino UNO, I had to find an easy way to mount the UNO board on any Lego constructions. I wanted this 'attachment' to be as light as possible. Here's what I did:
I did that by attaching Lego Technics parts to the UNO board with bolts and nuts.
This simple system enables me to configure the spacing between the Lego pins. This is done by sliding the back pins along the black bar and by rotating and/or sliding the front pins(with the grey bars).