Mate I don't think I would stress about the gap being any bigger or smaller.  You can always twist the wires around once or twice which would make it harder for them to fall out.  You could have 2 designs to suit different gauge wires but honestly I don't think it would be worth the hassle and confusion about which size to get.  One size fits all is probably the way to go.  Get the wires sitting within the restraint neatly and they shouldn't come out.

I will have a look at my other machines and see where else I can fit any.