I have faced several of these types of transitions. If you have some of the original flooring, you could try to meticulously trim/fit a plank and shim it to the level of the floor then secure it down with construction adhesive, but I've found trying to make it "invisible" is difficult, and matching the gap, with its imperfections, is a meticulous process.
Next best (if you're good at matching staining and mid-level woodworking) is to take a plank and cut it into a threshold that fits relatively tight to that gap with some overhang onto the flooring, stain/varnish, then secure it in place. Finally, you can look for a store-bought transition that would do the same function as a custom built one - it'll likely not match as well, but it would certainly fill the gap.