Repair/replace foundation vents in wooden siding?

The vents into my crawlspace are cut into the wooden siding and I’m not what the correct way to repair/replace them is. All of the examples and products I’ve seen show the vents in the concrete wall of the foundation, but mine are not. My vents are also smaller than what seems to be normal.

Here’s an example of what the vents look like. You can see the wooden frame is rotted.

After removing the vent, you can see a sheet of cardboard that has insulation above it and the crawlspace below. The wooden beam along the bottom is a 2×4 sitting on top of the concrete foundation wall.

You can see from this photo that the siding is wood.

All of the replacement vents I found were for sizes like 16″ x 8″ or 18″ x 10″. The portion that’s cut from the siding is 16 1/14″ x 7 1/8″ and the inside is 14″ x 5 3/4″. I didn’t feel like cutting a bigger hole, so I tried making my own frame from a 1.5″ x 1.5″. I’m not a big fan of the result; I can see into the crawlspace pretty easy which makes me wonder if water can get in easily too. The old vents had the horizontal boards slanting downward; I’m wondering if I need those too.

I guess I’m wondering what the best way to tackle this would be. Should I just make new frames similar to originals? Should I cut bigger holes and find fancy automatic vents that will fit; if so, how would that work with the siding? Are the horizontal boards slanting downward functional or are they purely cosmetic? I’m in the Seattle area, if that makes a difference.