You are going to need a 15mm length cut from the straight shaft of the skewer. If you Robovac model differs slightly from the G30, you can double check the measurement by measuring across the two halves of the hinge when the clam shell is closed.

A sharp knife, such as a Stanley knife, will be perfect for cutting the skewer. Make sure that you cut on a suitable surface such as a kitchen chopping board, and be careful that the cut end doesn't "ping" off into space.

Note: You can, of course, use a hacksaw or rotary tool with a cutoff wheel, but in fact, the knife has the advantage of slightly crimping the cut end, making it easier to insert.