I had bought a box of plastiline clay to make some "dams" to keep the fiberglass flox from going where I don't want it to go. Clay works well as the fiberglass won't stick to it and it can be molded to where you want it to be.
I made a dam where the notch was going to be. I made another dam where the upper lip of the fairing was because I wanted to lay a little glass along that edge to fill in the gap a little. After putting the clay in place I put the skirt back on to press the clay in place and form it around the rail section where I made the aluminum cap. I then removed the fairing again, inspected the clay and removed a little here and there till I was happy with the dams.
I then prepared to mix the fiberglass. I have cotton fibers and milled glass fibers and decided to use the milled glass fibers to make the notch and fill in the top lip. I filled the areas with the flox and when I was happy with it, I set the skirt back on and clecoed in place. I made sure I had plenty of fiberglass to fully form the slide rail notch without air pockets....yet not too much that it would blow out the clay and leak out.