The normal standoffs won’t fit through the holes in the pibow ‘layers’, the holes are only big enough for an m2.5 bolt. Making the holes big enough would mean several bits of layer would fall off, or at least become very flimsy.
The coupe case in particular would otherwise be a great case to use with HATs. Is there any way to make a standoff that will wotk with a Pi 3 and a coupe case? If not, it looks like the only option is some robust tape over the hdmi and audio sockets to prevent bits of the hat shorting out.