There's several vendors of trunk mats out there, but I suggest using a piece of automotive carpet instead. The trunk mat collects moisture under it, especially in a convertible.
I bought a roll of cheap loop carpet on ebay and cut it using the old mat as a pattern.
The trunk "dress up" option that was available had a black carpet, so you aren't too far from stock.
Bought mine from one of the many vendors that had them at Carlisle.
Grey plaid and matching spare cover. I just roll up the mat while the car is stored.