I have a WSM 18.5", but I ran lower temps a different way, first I use lump, but I believe this would work with briquettes, maybe better. I stacked lump around the outside of the charcoal ring, making certain they were touching one another, I lit one end and adjusted my vents for the desired temp, I have smoked Jalapenos at 130°-140° for 3-4 hours and I have smoked cheese at 80° for 2 hours.
I was surprised at how little charcoal was needed for the lower temps, I tried for low temps with the charcoal ring half full and just a few lit pieces, but the temps went to 180° within just a short amount of time, there was too much fuel available for combustion and I couldn't control how much lit because the fire spread out in all directions, with the fuel stacked in a line I could control how much ignited, then I closed my vents to have the necessary air flow for my desired temp.
I was wanting to hold a steady temp, but if you were to increase the number of briquettes (more briquettes burning at one time) in the line, say 3 abreast in the rows, that should allow you to raise the temp, then just continue to add that number of briquettes in the line as needed going around the circumference of the charcoal ring, until you need to increase the temp again and then add more briquettes to the line, say 4 abreast in the row.
The WSMs are great at holding temp, all that is necessary is to determine how much fuel is needed to attain the temp we want, it took some messin' with at first, but after I determined how much fuel to place in the line it was pretty much set and forget like when smoking at the higher temps.
One last thing, make certain there isn't any ash under charcoal grate, ...that gave me problems at first.