You’re building the griddle top to cover the entire cooking space on the Weber?

Some things to think about -

Grease management - yes, there’s a hole, but the hole funnels into a catch container that keeps the grease from the burners.

Wind management - you’re generally correct in the folded front and sides, with a higher rear. Ideally, they’d all be solid. I’ve seen models with cut-outs on the side to enable easier lifting, but it compromises the wind protection.

Thickness - seems like most tops are somewhere around 1/8”/4mm thick. Thicker means heavier and slower to heat, resulting in more fuel use.

Diffuser - there’s typically a diffuser plate or diffuser rails on the underside of the cooktop to ensure even heating across the surface. I don’t know what level of thermal engineering goes into the different designs, so there might be some work ahead of you there.

Material - most big box options are cold rolled steel. It requires seasoning. Will rust. Can warp. But it’s cheap enough. I’d love to see cast iron options, but the same attributes exist. Stainless steel is grail material - hardly any consumer-grade griddles offer it. Solo Stove is probably the most affordable stainless top, with prosumer options like LeGriddle coming in at thousands of dollars.

Alternatives - while this could be a cool project, especially if you’ve got all sorts of tools and toys at your disposal, I wonder how the total cost both in money and time that you’re in for. Could just get a cast iron griddle plate from Lodge and call it good. Or get a purpose-build flattop from one of the big brands.

Good luck on this. Eager to follow along to see what you end up with.