some people have put sandbags on top of them. They cover them in a cloth so it doesn't looks quite as bad, but it still is pretty eyesworing. I have seen people put they're heavy amps on top of them. Mayber a paving stone or something of the like. Use the imagination.
My sub weighs around 200 pounds so I have not had much trouble with it moving around . One thing to consider when putting your amp on top of the subs is that the enclosure vibrates, so it might shake something loose in your amp. I had my amp on top of my sub and it buzzed.
Where can you get a custom Good looking ROCK SLAB that weights 20-30 lbs? And how much do you think it will cost. My sub weights 74 LBS, its no light weight, but the high excursion drivers shake it pretty good. Any thing i put on top of it slides right off.
I used spikes to stop lateral movement. My current subs are extremely heavy (over 200 lbs each) so they don't hop around that much. You can get granite slabs from any counter top installer ($$$). In the past I've built a squat wooden box and filled it with play sand. Properly finished this blends into the room nicely.