where to put the prefilter, or really how to construct it (materials, etc.). What material should I use for the drip plate?
Depending on exactly what you are using for a sump i.e. a trickle filter already has a driplate and basically you would want to fashion something similar. If you use a fish tank for a sump you will have to make a driplate out of Plexiglas. Basically its a square or rectangular box with hole in the bottom & sides & ideally with a small strip as a anti splash guard.