I've seen plenty of them built from 30 gallon drums but I've never seen a 15 gallon. That's not to say it wouldn't work, I'm just not familiar with the dimensions of that size drum.
Most I've seen are 14in diameter. I made 1 for my folks that size since its just the 2 of them. Works great for shorter smokes, chicken, etc. You are a bit limited with the size so a smaller fire basket means shorter smoke times. The Weber Smoky Joe mini kettle grates fit and are $10. I picked up a cheap tabletop clone from Walmart for $15 mainly for the domed lid to get some more height. Dollar Tree had these cheap pizza pans that I used for a diffuser, def need that given the short height. I put in 2 racks, just use the bottom rack if doing a whole chicken or butt.