Mobile Set top box (Car Set top box with sync!)

I have spent hours the last few years trying to get something to work!! I have my media digitized but still have to dig out dvd to put in the car which SUCKS! with plex sync features i keep trying and not getting anywhere because of software/hardware limitations.

My vision would be to get a set top box (Roku, Firetv which all have usb port for storage!) Mount it into my car. When my car is parked at home it would connect to plex and sync videos to it for on the road viewing! I do not want to have to stream them i want to sync them to a mobile set top box that i can put into the car and at a hotel take it with me! I have spent hours rooting my firetv in hopes that i can do it but with the new app the tv interface has NO SYNC options??? Rasplex NO SYNC options!!! , Roku NO SYNC options!!!

The newer phones are not supporting hdmi output because they want you to do wifi mirroring i DO NOT WANT TO USE OBSOLITE DVD IN MY CAR they get scratched and lost!!!

Anyone able to figure out a way to do this without mounting a full computer in the car? I want a low power device!!

Thanks for any help!