I'm going to make an oak threshold for an exterior door. I know it's not all that expensive to buy one but for lack of a better explanation - I just want to do this. I've been looking online for a vinyl insert and all I see are replacements for aluminum thresholds. Basically I think I need an insert with a simple rounded with two ribs that I can put into slots in the oak. I hope this makes sense. Have anyone ever seen something like this? Thanks.

 

BTW - I've included a picture of what I have in mind as an attachment.

 

Looks like the same insert I have on store bought threshold. Google this
oak threshold insert
Came up with a page of them.

 

Another solution would be to put the seal on the door bottom instead of the threshold top. But your design will work just fine, provided you calculate the distance from the bottom of the threshold to the top of the seal correctly and the door doesn't bind or pass over it.

 

Far better to have that seal on the bottom of the door!
Even the factory made thresholds like that have issues with the seal wearing out and coming undone.
How level and flat is that floor in the opening?
What's worked well for me is installing an adjustable threshold and adding a door seal that just slips over the bottom of the door.
It's a brass color and has a small drip cap built in on the outside to keep water away from the seal.
Bought them at Home Depot.