How to mask in tight areas

Hard to tell without seeing it, but how about painting the bottom, then masking that & painting the sides? Then you've only one lot of masking tape to deal with and, if the bottom is wider than the sides are high, it will be easier to work with.

Not 100% sure of the scenario,but sounds like I would spray the floor and carefully hand paint the wall without masking.

How wide is the channel web?   Is the piece straight?   How long is it?

Paint it all the bottom channel color, then My approach if straight might be to cut strips of masking tape at about 3/4 the width of the web  (Straight edge & new xacto blade).  Apply one piece along the bottom with the edge of the butted along one raised edge.  Apply a second strip on the other side overlapping the first in the middle.  Spray the second edge color.   Remove the masking when dry & touch up

If the tape is wider than the channel you have two choices.  If I use wider tape, I find running a sharp toothpick along the corner usually will hold Tamiya tape into the corner.  Otherwise you can cut the tape to width first. One trick here is to cut the tape narrower than the bottom of the channel, lay one piece along one corner and the second, overlapping piece, into the other corner.

 

 

Maybe try a liquid mask? I use Microscale's Micro Mask with lacquers and enamels. Works great and keeps forever. If you go this route make sure to use a mask that's compatible with your paints, this product isn't for use with water based paints. Once it dries, which is fairly quick, you can trim the edge or whatever shapes you want with the tip of a sharp blade and remove the excess.