That cowl access plug on my 87 was a bit stiff and warped from being heat baked for 25 years in a sunny California environment. Still usable though. I applied some rubber conditioner to it and some RTV sealant in the lip to better hold it in place.
I would like to bring to your attention another seal on the bottom of the cowl cover on my 87 that I have not seen elsewhere. Originally it was a foam tape seal that was baked and crumbling into powder when I removed the cowl cover. I replaced it with a 1/4 inch thick by 3/4 or 1 inch wide low density foam tape weather seal. (pretty sure it was 3/4 inch wide). I'll show a photo of where it goes.
I think that the original purpose was for sound insulation, between the engine compartment and the open cowl area, possibly reducing noise into the cab, since sound insulation was something that they were focussing on in 1987. It could be a weather seal to prevent wind driven rain from running on the underside of the cover and running down the firewall.
Unfortunately I didn't capture the passenger side in the photo. It is similar but there was a break or a gap in the tape seal somewhere on the passenger side. If you decide to use such a seal, just tape it up approximately as shown, fit it up and if the tape interferes with an accurate fit up then just cut that part of the tape out.
Here you can see the dark line from where the original seal was taped on and I made white dots on the image to show where the original seal was adhered.
LOCATION OF FOAM TAPE WEATHER SEAL UNDER COWL COVER - FOLLOW WHITE DOTS:
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