I have in the past used my hands mainly, for a straight line such as a horizon, I use a sheet of paper held between the lens and board, and for smaller shapes I use another piece of paper with the approximate shape of the object or area I am burning in marked out then cut out with a craft knife.

If there is a horizon with a complicated line between the sky and the ground, I lower the head so that it is approx 8-9 inches above the board and draw an approximate outline of the the blurred image on a sheet of paper placed on the baseboard and then cut it out with the craft knife. It does not have to be that accurate. I then return the head to the original position, re focus and make the exposure for the lighter negative areas. When I am making the exposure with the shaped edge, I hold the paper mask approx 8-9 inches below the lens and this approximates the shape which I am trying to burn in.

Simple but quite effective.