About to replace electronics and need an idea on what to use for the hole in the console that’ll be left from a 5” Lowrance.

Find an office sign shop and have them make you a plastic laminate plate to put over the hole.

If you need a mount for a new bigger graph check with Bass Boat Technologies.
https://www.bassboattech.com/

I use white or black plastic cutting board cut to fit. Amazon

I just filled in the hole at the bow on mine with a black cutting board from Walmart.

The cutting boards generally do not have UV inhibitors in the material like synthetic marine board. Depending on how long it's out in the weather, cutting board material may be more prone to chemical deterioration.

I used this supplier recently. They can custom cut and round the edges if you want. Shipping was pretty quick. I believe they have gray, black and white colors. https://www.boatoutfitters.com/cut-to-size-white-white-king-starboard