If you're not doing a bunch of cuts like this, I'd just clamp a level or some kind of straight edge to the board and use a circular saw. They make aluminum clamp guides specifically for this as well. I have a cheap one from Harbor Freight that I have no complaints with. The level works better on really thin stock that might compress under the purpose made aluminum saw camp guides, as they want to squeeze the wood.

Kreg also makes some circular saw jigs for this type of stuff that people seen to like. But I've never used them.