Part F42 is a protective cover for the loader's periscope. Two different covers were used here on the Tiger E. One of them was associated with the low cupola, and one with the high cupola.
Tamiya have provided the wrong cover in this kit. Therefore you should not use this part.
Should you wish to build a correct part, the dimensions are here: http://tiger1.info/EN/LoaderPeriscope.html.