Infuse two sides foam at once

Wet layup and vacuum bag? there is risk with this as well.
To me it looks like you have plain core, no perforations and no grooves? if that is the case might as well do the wet layup and vac bag.
You will still need to perforate the core, don't use a drill bit use a plain nail in a drill. The hardened stainless steel tip from a telescoping antenna is perfect for this, grind the tip to a bullet head and you can do multiple sheets in one go this way.
Holes at 50mm spacing is sufficient.
I would not use tack spray either if that's what the spray can is, you may end up with creases in the bottom peel ply.

As you said that this is basically a shade cover and you want it light, 600gsm laminates will do the job.

 

Yes the material I found is plain... perforated and grooved can't be bought here as single sheet... only 50 sheets pack... would last forever!!

I drilled holes every 100mm... lots of holes... didn't think about the antenna trick, I will add the remaining holes this way to get to 50mm

I tried it on a small area but found messy, so will not use it

Perfect, thanks!

As soon as I will have a free evening or weekend I will do the test on the sample panel, and will update the thread.