Wood Dowels Over Expanding Foam?

[Pyrotechx]

I am going to vacuum form my visor for my Rinzler helmet, but I am curious as to how I should reinforce it from behind to withstand the suction from the vacuum. I was considering expanding foam, but I am not sure how this would react to the EVA, hot glue, or something else on the visor. I was considering buying some wood dowels and cutting them up and gluing them onto the back to reinforce it.

The piece to the right of this pic is what I'm wanting to reinforce, but I am not sure if expanding foam like "Great Stuff" would expand too much causing it to warp or crack.

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[laellee]

Expanding foam has a habit of deforming stuff that isn't already reinforced. One option is speedset drywall compound (not spackle or putty). The fast-set stuff comes in powdered bags that harden in usually times of 5,20,45 and 90 minutes (each bag is labelled for cure time). You can mix it with water to whatever consistency you want, at it'll get rigid like plaster of paris. It's low shrinkage, but it will add a bit of weight (not necessarily a bad thing).
I used to use this all the time from finishing drywall when I was younger, and found it carries over to a lot of prop projects nicely. Costs around $10 for a 20 pound (or so) bag at Menards, Lowes, Home Depot, etc.... One bag would be plenty to reinforce that. FYI it also doesn't superheat like bondos, which causes another batch of problems.