I now think that Primus LiTech will be the pot I should get. They are relatively cheap.

http://www.amazon.com/Primus-LiTech-Trek-Kettle-Gray/dp/B002BU4CHY/ref=pd_sbs_sg_6

I'll add some little item from Amazon for $1.50 to bump the total price up over $25 so I can get the free shipping.

If you scroll down that Amazon page to the user reviews, the first one (by user "Kennon") lists the dimensions that he measured. He says the pot base is 4-5/8" in diameter. That's 4.625". I measured my firebox inside dimensions again, very carefully, and found that the minimum i.d. is actually 4.7". So this LiTech looks like it will be the biggest pot I can squeeze inside the firebox, which is what I wanted in the first place.