There is no earthly reason to carry a spare bottle of DEF around. You can buy it whenever you need it at any fuel station that carries diesel fuel, as well at any WallyWorld, auto parts store, farm supply place, and thousands of other places anywhere you go. Given that all the vehicles I know about give you a “need DEF” warning a thousand miles before you actually are at any risk of running out, you are never at risk of running out.
As for cleanup after a spill, plain water is all you need. It is very soluble in water, less so in any sort of grease cutter or cleaning agent. Pull the rug out, hang it on a clothesline, and hose it down. Ditto the truck bed. Problem solved.