Looks awesome! One of the most annoying parts about eva foam is trying to sand off textures or trying to find good affordable foam with no texture (at least in thicker sizes). Did you make a pattern or did you just go by eye?
Probably more info than you're interested in below, but here goes. If nothing else, I'll have this recorded somewhere for the future in case I forget how I did this (quite possible).
I started with a roll of packing paper (brown fairly thick stuff). After convincing it to lay flat, I drew a straight line, top to bottom, the height that I wanted it to be. Then I drew out the shape of the shield by hand. Then I cut out the side that looked best, folded it along the middle line, traced the shape onto the other side, then cut the whole thing out.
I took pins and pinned the pattern to the smooth side of a piece of flooring then traced the pattern with a sharpie. Then I put the foam on a self healing mat that my wife uses for cutting out material for sewing, and carefully cut out the pattern with a box cutter.
I then took out a head gun, heated up the foam and put the bend into it that would give it that shield shape. Held it in place until it cooled down and it stayed pretty well. I've got some small scraps of 2x4s sitting on my table that I rest it on to keep it in that shape.
I took some of the brown paper and pinned it to the top of the shield and cut out the shape (could have used the original pattern but wanted to keep that in tact in case I needed to start over). With it still pinned to the shield, I started drawing the raised parts. Took a good bit of drawing, erasing, re-drawing to get it like I wanted (I'm not an artist). Once I had the shapes drawn, I cut them out and pinned them to the yoga mat that I had sanded down with a mouse sander. I traced them with a sharpie, then cut them out with an exacto knife on the self healing mat. Any of the pieces that had more than one of the same kind, I used the best of the drawings I had done for the pattern so they would match from side to side.
Once I got it all cut out, I laid it all out on the top of the shield. That's when I couldn't decide which way to go and came here for help.
Once I got the advice to glue it all down first, I started with a hot glue gun and glued it all in place. Being my first time working with a hot glue gun, I'm happy to report that although there was a decent amount of pain involved, no lasting injuries occurred.
Then it was back to the heat gun, heated up the worbla while wearing my heat resistant gloves, fitted it over the whole thing using a dulled down plastic knife to get the fairly sharp edges, then let it cool back down.
I laid down some newspaper in my front yard, put the shield on it and painted it with the spray on primer. About 45 minutes later, resprayed with a second coat and let it dry for about an hour. That's when I noticed the pattern of the yoga mat peeking through to the surface.
I picked up some gesso last night and plan on putting many layers on top of where the pattern pokes through, then sanding it to make it smooth. Hopefully I'll be able to get away with just doing that in spots instead of the whole thing.