Are they on stiff cardboard or are they like a fabric or something? I would just safety pin them onto the corner. When you do your dressage test which way do you turn at C? Attach them to the opposite side. I.E. If you track left attach them to the back right corner so the judge sees them as you turn.
sew velcro onto the saddle pad and get stick on velcro for the number. I see alot of people do it around my area, they usually use the bridle number discs and cut off the elastic then velcro it onto that saddle pad. It's cheaper then buying one with a number holde on it and you can remove the number disc when you wash the saddle pad!