If you plan on sanding it all you can use wax paper over the plex and if poly you can add some special resin wax when you mix it up. That will make it easier to sand and tack free when the lay-up dries.

If you are doing the edge of a dagger board I've used duct tape to make a temp mold. I've used the silver side of duct tape against the backside of where I wanted to make the repair just slightly larger then the repair area. Then taped it place with another strip of duct tape larger to hold it in place sticky side to sticky side. I've waxed the silver side before but usually it just pulls off and the fiberglass does not stick to it. If it does a little paint thinner or similar will loosen it.

I hope it gives you a couple of ideas.