Stupid Drywall Question

So I'm doin some work on the house. I hate sheetrock and drywall, but that's where I'm at. Removing all the batten so the inside won't look so trailerish. So I'm in the rear bathroom and Have gotten a couple of the flat joints done pretty good. Actually dang good for me. lol I'm in the corner behind the door. It's such a tight corner, my corner joint compound spreader wouldn't fit, so I ......you guessed it, smoothed it on with my hand and fingers. I put fiber reinforced sheetrock tape over the corner first and then joint compound. So is there a sanding tool that'll fit the corner or am I just stuck with a sanding brick?

..and yeah, I've looked online and didn't really seen anything that I thought would work.