If you’re playing on PC, the most recent patch cranked up the rate of Bevelros occurring (it’s almost guaranteed now, iirc). If you’re not on PC, console patches take longer to get released (because the consoles have much longer verification processes for patches and oftentimes limit them to once/month) so it should be coming sometime soon.
Of course, that’s not the only way to get the Shade of Bevelros. If you do the Infinite Arena, Shade of Bevelros is one of the enemies that can appear. It might be easier (and a lot more interesting) than repeatedly running Iron Outpost and praying for luck.