I think that the cover for the 1947-1955 lower-right, inner-cowl opening is: COVER, right inner panel, part number 3684403.

Mine measures 11 3/4 by 6 7/8:

Here are two photos of the cover - a merged image of a flash & a non-flash photo.

The cover is attached with six 1/4"x 1/2" clutch-head machine screws (and "star" lock washers) into captive nuts in the inner cowl.

Tim