A couple suggestions:

Bring a primary through the firewall (8-10ga wire) with at least a 30-amp circuit breaker immediately between the battery and the lead. Inside the firewall install a auxiliary fuse panel or two.

Draw your power from the fuse panel. This reduces the number of leads going through the firewall and puts circuit protection on the primary in case it shorts somewhere between the fuse panel and the battery.

Even if your fuse for your device trips, hot power to the primary can still burn your car to the ground if it is not protected. The more you have running through the firewall, the more risk you introduce.

I put two auxiliary fuse panels in - one switched with the key (using a relay) and one that is "always on". Power for both comes from the battery through the circuit breaker and then distributed inside the firewall to the panels.

Here's the circuit diagram:

Parts list is on my site (click here)