I have made a lightweight custom 1x10 di2 for mtb.
I have a completely custom rear harness with 26awg silver wires, heat shrink tubes, and custom battery plug (JST).
Front haress is stock Shimano, but I have changed the plug to work with Sprint shifters.
Weight:
Front harness (contol box) with Sprint shifters: 23g
Rear harness: 29g (shimano stock is 80g)
Battery: 16g (shimano stock is 70g)
RD: Shimano Di2 with Fibrelyte pulleys: 206g
Total: 274g
Works great, shift down with right hand, shift up with left hand. Battery is 250mah, but works for a long time, because it's the front derailleur that really uses a lot of energy.