I've got the original undertray on and hardly ever get anything under it. Did you cut the front corners off?
All you need to do is trim down the sides on the rear corners, once its all screwed on its a pretty close and sealed fit. Our tracks in the UK are smooth astro but i've raced in seriously wet conditions and the insides of the car have been bone dry! only a little bit of crap collected under the tray by the open slats.
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