Motion Labs and Lex (and others) offer rack mount distros for L21-30, L14-30, and CS-50 inputs. There are also industrial/construction "spider boxes" for L14-30 and CS-50 inputs, and various "lunch box" styles to chose from.
All of those would arguably give you a one cable run from your main distro to each side's smaller distro, depending on the expected real world amerage needs for your rig.
Look for UL or ETL certification on whatever you select; it will make the AHJ happier (and also your insurance company). Buy once, cry once.