I've got a few friends who carry their tracks on either MP3 player with a good playlist manager, or an iPhone/iPod/iPad. Just put your MP3 backing tracks on it, plug it into the sound system (stereo mini-plug to stereo 1/4" or whatever's appropriate -- you might want to keep a mini-to-mini and mini-to-1/4" with the MP3 player at all times), open your player program, and click Play.
Best,
Tom SmithWin10/64 • i7-8700K • 24 GB RAM
BIAB 2025 • Reaper 7 • Izotope MPS
AKG C1000S • IK iRig I/O Pro