Reflectors can be made out of foam core boards, like those sold at office supply places, or art supply shops. White works well, but silver foil or silvered contact paper can also be glue to the surface on one side. Manfroto makes some interesting "frog gig" or "pitchfork" type holders that penetrate into the foam and have attachment provision for grip equipment to help position the boards. Other options are things like car windshield sunshade dealy-bobs, or fabrics stretched tightly on PVC pipe or wooden frames (look up 'scrims' on Google). A scrim frame and a reflector frame are basically the same thing; the scrim with a semi-transparent fabric, the reflector an opaque fabric that will reflect light; and with a black fabric, it can be considered a "subtractive reflector" or a "flag", to "flag off" stray light.
My biggest tip is that while round, sping-open wire-frame reflectors are popular and portable and low-cost (look for 5-in-1 Made in China ones to see what I mean), square or rectangular reflectors have some advantages as far as placement and positioning; they can be plopped on the ground, and the bottom edge will stay put, and if a second frame is clipped to the top, or side, the reflector will stay in place pretty easily as long as it is not windy.