Not real sure what 20 Ga configuration your looking for but I have a Springfield/Stevens M 94 single barrel you may wish to consider. All steel and walnut, and is nice and tight.
2 3/4" chamber, 28" full choke ejector bbl. More pics if interested.
See below. $160 Shipped to your FFL.
Edited: Whoops, it helps to read the whole thread. Disregard. I see you're not just looking for any single shot 20Ga