replacement wood threshold insert for metal door
Too late for the original poster, but for others who find this with the same issue - looks like someone has them now:

https://betterdoor.com/shop/exterior-door-sills-thresholds-replacement-parts/endura-composite-z-series-adjustable-cap/

https://betterdoor.com/shop/exterior-door-sills-thresholds-replacement-parts/endura-oak-adjustable-cap/