I strongly suspect that taking your time to make the foil as smooth as possible would improve the quality of the signal.
It probably doesn’t need to be very smooth. WiFi, depending on the technology, uses 60mm or 120mm wavelengths. Those wrinkles will be less than 1/100 of the wavelength, so they shouldn’t have an influence. Getting the shape right is the tricky part. Antenna design isn’t trivial.
I would start with a few WiFi SSM’s, spread them around the house (target locations) and then experiment for a day of so. A sheet of foil held up by two sticks, themselves stuck into globs of plasticine. Move 'em sticks and map the response at the target locations. Eventually you’d get a sense of it and would start optimizing, me think.
And I certainly would not worry about the legality of the set-up, there is no way you would get any kind of gain that would suddenly put your registered/tested WiFi router in the Canadian-FCC-equivalent’s bad book. Plenty of losses, but gain? Nay. Targeting is the objective.