File attached;
In the pictures you can see the cavity in the blue aria needs to be filled up completle
Another option would be if anybody has an native Solidworks file of an 2011 STI/STV frame that has all the features,
Thx Ton

One more picture

Its for a friend that lives in a country high cab magazines are
not allowed for civilians.
He wants a single stack magazine in the 2011 frame as i have a bunch of polymer grips for him, so he can swap between polymer and metal gripps

Your files are from future version, i can't open it. Try to cut from step (see attachments) or cut a piece of body to fill original model.

This better not be for a gun !! LOL
"Delete Face" is your friend, from pictures it looks like it is already surface body
Delete all the faces that are wrong, patch up all the holes left out, make sure you knit all the faces and turn it to solid and then you can sketch and model the cavity you need.
ps. This may not be as simple if the file format was .stl