If you have asked Optus and they don't supply it, an engineering student in Adelaide appears to make them on a 3D Printer - https://www.thingiverse.com/thing:3771562 . His details are viewable on the Nortronics link.
eBay also has small adjustable shelves for set top boxes and other small electronic equipment with cable slots that may also be suitable.
Unfortunately, we don't supply wall brackets/mounts. As for the advice you were given regarding outages, I don't think that's correct. If anything, mounting the modem above ground level should positively impact speeds/signal stability.