Pretty much any 20A automotive relay connected to the battery via a 20A fuse and a a cheap terminal block of your choosing will provide the required functionality and provide switched power (cuts off when engine is turned off). Total cost
Note all automotive relays have the same pin numberings marked on them.
Pin 30 - connects to pos battery terminal via 20A or less fuse (high current)
Pin 87 - switched distribution power source (high current) - to terminal block
Pin 85 - relay trigger ground (low current)
Pin 86 - relay trigger positive - wire to any switched 12v source like running lights that are only on with engine running (low current)
Suggest using 12 gauge wire for high current wires (it is good for up to 40A) which is more than will ever be needed. Any thin wire wire can be used to trigger the relay. Also solder rather than just twist all connections and wrap with electrical tape. When soldering heat the wires to be soldered until solder touching the wires (and not the soldering iron) melts into the wires. Installing a 20A fuse between battery and relay is a good idea. Twisted wire connections tend to be a source of power loss and possible failure point.
I would say on a 12 volt system it is good for 15 amp maximum, not 40 amp. here is a table used for sizing emergency lighting wire size, as a example, you have a 250 watt load or 20.8 amp @ 12 vol, maximum distance is 8 feet ( 16 feet total ) with a 5% voltage drop, having said that, in general, number 12 should be good for just about anything on the V: