fermenter size for 15.5 gal keg

[SirPuFFaLoT]

Hey guys, what size fermenter would you suggest to get a full charge on a 15.5 gal keg using UJSM if I rack to the top of the grains?

I am currently using 14gal barrels which hold a 10gal UJSM wash nicely. I get 2gal low wines from a stripping run and I leave around 1 to 1.5 gallons with the grains after racking. Side Note: I am still adding 2 gallons of backset but I have to add a couple teaspoons of NaOH to get the Ph up to 5.0.

I am thinking that a 15gal wash would be the perfect size to get a full charge (12-13 gal) if I rack to the top of the grains. Do you know if a 20gal barrel will hold a 15gal wash of UJSM? The next size up from 20gal would be 30gal but I can get the 20gal a bit cheaper. The barrels I am looking at are all blue open top HDPE drums.

EDIT: I plan to save my 14gal barrels to use for future 1-off recipes (perhaps when I make a boka).