This antenna:
150/450/800 Antenna, Gain (dB) Unity, 100 Watts, Whip Length 16.5"
theantennafarm.com
or this one:
EM-M43002 Tri-Band IP67 Rated Roof Mount Antenna, 150-162/450-490/763-870 MHz
theantennafarm.com
Both Larsen and EMWave are well respected companies that make professional antennas and don't screw around.
You could install an NMO mount on a cookie sheet and have an easy ground plane.
Or, you could purchase an NMO base adapter like one of these:
Mobile to Base Station Adapter Kit for NMO Mount Type Antennas. Includes bracket, clamps, radials and mounting bracket. Frequency Range up to 512MHz, N Female
theantennafarm.com
or
Mobile to Base Station Adapter Kit. Use with NMO style HF, VHF and UHF antennas. Includes bracket, clamps, radials and mounting bracket. Frequency Range 406 -
theantennafarm.com
Those will supply the necessary ground plane for the antenna so it can work to it's full potential.
Both of those base adapter mounts have type N female connectors on the bottom. Buy a length of quality coaxial cable with a Male N connector on one end and a connector to match your scanner on the other. The exact type of coax will depend on how long the run will be. I'd suggest something like LMR-240, it's manageable and has low loss, even up at the 800MHz frequency range.
Unless it falls off the balcony, it'll probably outlast your radios and the building it's mounted on.