you used to get right angled brackets with servos for mounting them but you dont get them anymore.
Luckily you have Sintra though...
Cut a strip of sintra twice as long as the tabs on the servo (the tabs with the screw holes in).
then drill holes to line up.
Finally (this works because you have sintra). Boil a kettle, and drop the piece if Sintra in hot water, leave it for a minute then carefully take it out. Now quickly bend it to a right angle in the middle before it cools down.
You now have a right angle servo bracket that you can screw onto the servo and the main Sintra base (it is a better connection then velcro, looks more professional and can be reused easily.)
addit: Sintra is expanded/extruded PVC with a thin coating of dense PVC on the outside, this allows it to be bent and shaped at low temperatures (from about 68 degrees if I remember)
Because of these properties, Sintra is liable to bend or warp if left or used in direct sunlight or high temperatures, so choose your application carefully.