I haven't seen an arched door way been posted yet so i thought i would share mine.
There is an single arched doorway and a arched door way with a wall attached.
If you can not see the prefab in radiant keep it high lighted and press the stamp button.
Arched Door
(https://i.imgur.com/UWXjAQn.png)
(https://i.imgur.com/XJLgAH0.png)
Arched Door With Wall
(https://i.imgur.com/aMug7np.png)
(https://i.imgur.com/YlskFt7.png)
(https://i.imgur.com/a2UwJoc.png)
Its just a simple door way nothing special aha but enjoy :)