THERE MAY BE A POST ON HOW THIS IS DONE I KNOW I READ OR SAW SOMETHING SOMEWHERE. IF I WERE DOING SOMETHING LIKE THE ETHIOPIAN SHIELD WITH BUMPS I WOULD MAKE A HUMPED UP FORM OF CLAY AND PLACE ROCKS WHERE I WANTED THE DESIGN BUMPS. LET THE CLAY HARDEN AND THEN STRETCH THE WET HIDE OVER IT AND LASH IT DOWN TIGHTLY. MIGHT NEED TO BUT STONES OR WEIGHTS ON TOP TO MAKE IT CONFORM UNTIL DRY.
JUST GUESSING A WOOD FORM WOULD WORK BUT BE MORE DIFFICULT TO MAKE BUT WOULD NO DOUBT LAST LONGER AND BE EASIER TO STORE WHEN NOT IN USE. THE SHIELDS I REFER TO ARE VERY THICK AS HIPPO HIDE IS WHAT THEY USE MOST OF THE TIME. HERE ARE SOME PICTURES OF THE TYPE OF SHIELD I AM DESCRIBING.