Yeah, I can confirm that trying to coat a pre-cut to size piece of substrate doesn't work well. You will have to get used to the idea of imprecision and some loss. On the other hand, I have cut leftover edges as small as 35mm single frame "sheets" for testing. Tape the small single frame sheet to a piece of black construction paper if you like to fit in your SLR. A nice thing about doing this is you dont have to try and make and run a roll of film if you are trying to decide on developing procedures, etc, and aren't using large format cameras. You could probably come up with some sort of paper film holder that would fit inthe back of an SLR. (Hmmm) Done this with 120 in my Yashica TLR as well.I wouldn't bother trying to coat a pre-cut material because of the problems of emulsion leaking out or not filling the whole substrate.
I now coat by fastening the substrate to a sheet of 1/4" thick plate glass. I have a few of these as leftover glass shelves from a display case. You can find something cast-off or I believe even new plate (window) glass is not very expensive. It's flat and holds heat well so your emulsion doesn't start to set up on you while you try to coat. (warm the glass a bit before coating)
To help make certain I have covered the area I need to coat without overdoing it, I have made up some paper templates for 120 rolls that I tape to the underside of the glass. Now you can see the template through the substrate and glass and make certain you have covered the proper area and try to avoid creating waste. I have templates made for one roll, two rolls and three rolls.