We have also used GRP and plastic boxes for indoor APs to be used outdoors and not seen any problems with signal strength. tested with the door open and closed without a difference in signal strength. if your curtain is large and heavy, there may be a metal mesh inside the layers for strength that you may have to consider. if it is there, depending on the mesh size, it may or may not have an effect. You know, just to muddy the waters
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