How to get masking tape to stick

 A couple of things that help. Don't try to mask too long a line with a single piece. Cut a lot of little pieces and match them up end-to-end.

Don't  try to mask around tight curves. Cut the curve in the tape, small sections if you need to.

Tape needs to have direct pressure applied to stick. If I follow what you are trying to do, maybe you paint the bottom of the channel, then press down tape on that and paint the sides.

I do some of my best problem solving thinking backwards.

Another idea....

Have you tried burnishing it with a little burnishing tool, or the big end of a flat toothpick?

Have you tried tamiya vinyl tape?