I have the a MP (film)... using a PL seems to be the easiest as I have not seen a PL without markings, but even if they don't... Just use two filters where you have matched the marks (or marked the filters in some way). One filter on the camera and one hand held.
The Lee Seven5 filter holder has markings on the holder... 1/2, 3/4, etc. In worst case align the filter with the appropriate markings and put the horizon at the same level in the finder.
No problem hand held in both situations...