Edit two items from 3D STL files

What I would like is this 3D project (https://www.thingiverse.com/thing:4106305) converted from a 4 x hard drive cage into a 5 x hard drive cage, like this one (https://www.thingiverse.com/thing:4106284) so the drives are evenly spaced out, as I don't need the large openings on the longest sides of the 3D model of the 4 x bay cage but keeping the same overall external dimensions of the 4 bay cage. IE: keeping the rounded corners and bottom step back as it needs to fit the Thermaltake V21 pc case space which the 4 x bay cage does perfectly.

Also to alter this 3D model (https://www.thingiverse.com/thing:4106276) by removing all of the centre content of the 3D model and leaving just 2 separate side rails.