Delayed response! Oops!
For raised details, such as flames, I'd recommend a closed cell foam. 1/8" or 3/16" would work best. Anything thicker would be a PitA to get the H/L material to conform to the shapes nicely.
Depending on how you want the flames to be on the H/L, (I'll suggest back from the indents for the visors.) you'd want the closed cell to be larger than the finished size.
Lay the pattern for the flames onto the closed cell. Do not use a felt pen unless those marks are to be cut off. Felt pen will bleed through to the fabric over time.
Once you have that marked, cut the flames on about a 30* angle with a razor knife. Cutting them @ a 90* angle will be too square for the H/L material to be able to fit nice & tight to the closed cell.
Try to make the cuts for the flames from flame tip to flame tip in one shot. Any hesitations or points of starting & stopping will create deviations in the line of the flame.
You'll need nice sharp new blades. For some reason, closed cell eats the edge of the blades. They get dull pretty quick. Do the cutting of the closed cell on the bench, not on the H/L panel.
Once you have them cut out, lay them onto the H/L panel & trace them out onto the panel. Regular school type blackboard chalk, sharpened to a point for a nice thin line is what I use.
Spray glue onto the backside of the flames & let it dry. Do the same on the H/L panel inside & up to the chalk marks on the panel.
Once the glue has dried, spray another light coat of the glue onto the flames.
The, starting from one side, lay the flames on to the H/L, holding the closed up & away as you work across the panel as you lay them down, matching them up to the marked lines without pushing them down tight until they are all in place & your happy with where they are. You'll find it's easier with a 3rd hand to help you. Those tips of the flames will want to flip & flop around until they are laid down. Once that's done, then press it all down to make the glue adhere.
To make a nice transition from the flames to the visor indents, a piece of 60 grit sand paper can be used to make a nice taper leading up to those. Then give those flames a once-over with that too. This'll give the H/L material something to really glom onto.
Then it's just a matter of doing the recover the same as a regular recover, but paying special attention to get the H/L material to snuggle up tight to the shapes of the flames.
Hope this isn't too confusing! Any Q's? Ask away! Good luck!