I'm using stainless steel marine vent (Clam shell style) to cover the hole. Power connectors are Trac https://www.tracoutdoor.com/shop/ele...onnector-kits/
And 16pin connector for RCA input, trigger and speaker output

 

That's a good looking setup. I like what you did with the different holes.

 

Thank you. Here is closer look. Nut and bolts at the left side holding the amp bracket. Stainless steel vent placed with an angle so water will not go through this hole and also secured with nuts and bolts

 

So I am late as always but pics as promised. Let me know if you need something more specific.