I made these myself a few years back. My advice is to either make it yourself, or buy the oversize one and trim it back. To trim it back, use a flush cutting router bit, piloted on the bottom. BE CAREFUL of the top, as the router bit will fall into the vent holes if they are not covered, or your pilot cannot be adjusted to not hit the vents.
Added some pictures of the bit. I used some Melamine tape I had laying around to cover the vent holes.
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