These are not a stock item but Joanne can make one up for you, includes;
Suitable for your modern carriage (2 or 4 wheel)
Tool Roll
Adj. Spanner + spanner
Allan Keys
Hole Punch
Lead Rope
Knife + Scissors
Rein Coupler
Bungee Strap + cable ties
Hoof pick
Emergency bandage (horse)
Bag to keep spares kit in
Now in theory you should have a replacement trace that fits your harness!
Contact Joanne if you would like to purchase one of these spares kits.