You also may want to have a strip of cloth connecting the top of the blanket to the top of the cross section, so you caparison does not go sliding off your horse's rear as he moves. Use heavy cord to connect the caparison together. Either set eyelets or use the button hole stitch to finish the holes for the cord to go through. Rein-covers need to be about a yard long. It is better to make one for each side, rather than one long strip, so you can control the reins directly instead of having them slipping underneath the cloth.
Paper binding clips can work to keep the rein covers in place.
Caparisons are a wonderful way to deguise mundane tack and add heraldic flair to equestrian events. They also can dramaticly dress up a less than attractive horse. Caparisons can be as simple or elaborate as you wish. Note the two books mentioned at the beginning of the article for starters. Good luck, happy sewing, and safe riding.