Even if we ignore the possible legal trouble of modifying street fixtures without permission, building anything that can hold up out in the wind and rain is going to be a big task. Getting it up there safely is going to be a challenge. Making sure it’s secure so the next puff of wind doesn’t send it down on someone’s head, maybe even yours, will be another challenge. I’ll make a recommendation for ‘blue tack’ and some heavy cardstock or plastic in front of the windows. The blue can hold for years, but leaves zero marks, and the right material could be chosen to allow whatever amount of light through, anything from all to none. Project boards like kids use for school presentations could be great. They can be cut easily to whatever shape the window is, painted or drawn on for decorative purposes, and even offer a little bit of extra insulation.