For firewall passthrus I copied location from other builders and guidelines in the plans that say you'll probably need two holes for wires to pass thru. One is for engine wires and the other is for transducer wires.
I bought the Avery Pass Thru kits from Spruce. They were $100 each. There is another kit at Spruce for half the money but I didn't see it before I had bought the Avery. But these are nice.