When I built my backyard forge, I thought I might as well go to the expense of plastering it, since the walls are of concrete blocks. I had not previously attempted a whole room, having only had to do some patches around the shops and an unhappy experience of a ceiling when I was young. I got on all right, and I think it was better than a cowshed finish (as I was informed, thanks John) But needing a corner trowel for just that once, I used a prism cut from a piece of pine. ( The handle is a marimba bar of rosewood, salvaged.)
That prism has had a prior purpose. I was teaching myself handrail geometry. I think I understood the geometry eventually, but the handrail itself was not a success, The centre line of a handrail on an winding stair follows an ellipse, so the sides of the prism represented the stairsí pitch. It also has to twist, but I canít go into that now.
This is not a trowel, since it does not pick up muck and spread it!