The suggestions in this thread are excellent and I have little to add to that. All I'd suggest is hunting craigslist locally to see if you can't find something that you wouldn't have to ship since these things tend to be on the heavier side.
I sometimes feel it's better to leave the snow there to soak up the sleet/freezing rain/shit so that I don't end up with a skating rink. Since I'm not shoveling the weight of the snow isn't such a big deal.
There was 4" on the ground when I left the house this morning and we're scheduled for another 4+ today during the day followed by rain tonight. Tomorrow morning should be fun.
The diameter of the hole is only part of it. Assuming that the speakers have the typical embedded threaded insert you'll also need to know the # of threads per inch which won't be easily measured.