I've always used a "pump box" & been wfp over 7 years. I'm on my second one in all this time. Both were ready made by cleantech (who don't exist anymore).
The first one was fitted with bypass system & you controlled the flow via a lever (valve). After about 3 or 4 years it packed in. I suspect it was a simple electric problem & not the pump itself but anyway I just ordered another one.
This second one was fitted with a digital varistream & after a week of fine tuning & familiarisation made me think how I'd ever managed without it!!
Both have always been powered by the van battery via 12v cig' lighter socket but the second one with controller is MUCH easier on power consumption!
Mine are quite small, neat units & contain 100psi shurflo & strainer inside. The box itself is an electrical box (forget the name for them) but can be purchased from City Electrical Factors & the like for about £15.
Although I've bought my pump boxes ready made before, I will just make my own next time, they are very simple knock up. I like them because they are a neat fixture which can be easily taken in/out in winter.
So, do I carry a spare? No, I can have all the necessary parts delivered next day.