The microstrip filters I have tuned is done by using small rectangular pieces of pretinned copper foil. Cut some various sizes from a large sheet of petinned copper foil. A good start is to cut pieces having the same width as the microstrip conductors, but at some different lengths. Then use a plastic or wood toothpick and press a piece of foil to the microstrip filter conductors, while monitoring the bandpass response on a scalar or network analyzer. Try different locations of the tuning foil.
In some cases several pieces are required at the same time, this is tricky as most of us just have two hands to hold two ore more toothpicks...
When the best position of a tuning foil is found, just solder it into position. This procedure might require a pretinned microstrip conductor as well for achieving the best solder joint.
Crème de la crème is to provide Cu 'islands' (dots) on the microstrip substrate e.g. at some locations outside the microstrip filter elements, during design of the filter. Then one end of a tuning foil can safely be soldered to these 'supporting areas' and the other end to the microstrip conductor.