Step 1: The Parts
Spare piece of scrap wood
Earth Terminal Block (or a choc block of a decent amp rating)
2 self tapping screws
10 Amp Crocodile clips
Some 2.5mm wire, taken from Twin and Earth cable used in household ring mains.
Step 2: Customisation
Earth Terminal Blocks come in 8 way (16 screws) as well for even more possibilities.
Cost wise the Terminal block cost £1.34, the clips were 10 for £1.70 and the wood, screws and cable I had anyway. So total £3.04 vs £3.99 for the cheapest on Ebay with less clips and less flexibility.