Mortaring in the skimmer

I've been thinking about how to make sure the mortar that holds in and surrounds the skimmer gets into every nook and cranny if I make a form to hold it in place while I pour in the mortar. I figured out that I had to be able to place the skimmer in the form from the top because it was too big to fit through a hole in the form because of the flange. I even cut a notch in the opening to account for a small tab that kept me from sliding the skimmer straight down into the form. Instead, I would slide the skimmer down into the form about an inch from the face, then push it flush against the form over the tab. But unfortunately I did not think about how I would get the form off after the mortar was poured.
I had to break the form with my fingers to remove it, otherwise I would have disrupted the setting mortar and the position of the skimmer. But when I broke the form, the skimmer and the mortar moved anyway! I had a problem.
I must have worked that mortar and repositioned the skimmer for an hour to get it positioned correctly. Now I would have to hold the skimmer in place while the mortar set up to where it wouldn't move. I had to make sure it was level horizontally and vertically while maintaining 3/8" spacing from the wall for the future placement of the tile. I held and put pressure on it for over an hour, but it worked.