I have stock Pzero Neros on my car and roll empty trunk all winter. Just don't drive like a moron and you'll be fine.
More weight back there can also be a bad thing. You want more steering and brakes, not acceleration.
People like you are why winter tire laws are now here to stay.
OP: Cinder blocks. But typically, in winter time, I have enough junk in my trunk to equal the weight of any extra weight I would put back there anyway. Now's your time to carry your tools everywhere!
-2012 Grabber Blue GT 6M Premium w/ Brembos, Glass Roof, HID security pkg, Rear Camera, 401a, FRPP GT500 Axle-backs, Front Pioneer TS-D6802R speakers, JHR bracket, Bondra clutch extension.
-1986 Mazda RX-7 GX - Fast Mirage 16" Wheels, Sumo Firenza 205/60R Tires, slotted brakes + performance pads. Otherwise factory stock.
-2008 Mazda Mazda3 GT Sport - Stock, 5MT.
-2004 Volvo XC90 T5 - Comfort pkg, safety pkg.
-1995 Infiniti G20 - Sold, Pioneer head unit, lighting mods, miss it dearly.