I'd ask about the power supply you're using, and what that has built into it. From the sounds of it, we're trying to protect against user error, where you (or someone else!) sets the power supply to the wrong voltage. Why not swap it for a fixed voltage PSU that can only supply 5V?
For example
https://www.robotshop.com/uk/switching-power-supply-5v-50a.html
looks like it fits your numbers at first glance. I notice the voltage is adjustable over a range, so you might want to pop a bit of glue/thread lock on the adjustment pot
If you do strap you're own protection to the front of another supply, check to make sure they'll agree. Strapping a fuse to a PSU that has built-in Over current protection may not actually give you what you want.
Polarity protection, is much easier than current protection (IMO). You can do this entirely with physical things. Most beefy connectors can only be inserted one way round, then properly using red+black wires, PCB labels, and looking before turing the system on can be more than enough. If you (really) want to add protection to the circuit -
You can put a shotky diode in series that will block reverse current. You'll have to increase the supply voltage a bit to get 5V the other side (~0.3V) and you'll probably need to heatsink as it'll waste ~50Ax0.3V = 15W. It's wasteful but foolproof, as it doesn't matter what the supply does, neither the supply, nor the load can be damaged.
Or reverse bias it accross the supply, where it will conduct if the power is reversed. You then need to know what the rest of the supply is going to do when shorted, often you can use this to blow a fuse very quickly, or trigger the supply's built in protection. This does absolutely nothing untill the supply is reversed which is good, but something can be badly damaged if it's done wrong. The maths has to be done on fault current, fuse blow times, etc.