What's the best way to create a .cab file of multiple files quickly?

You can use the Windows built-in command makecab but first you need to generate a file list.

dir C:\FolderName /s /b /a-d > c:\temp\files.txt

This will list all files in the C:\FolderName directory and save as a text file with full path name. Now we are going to use the files.txt file to create the cab file.

makecab /d CabinetName1=test.cab /D DiskDirectoryTemplate=C:\temp /f c:\temp\files.txt

The above command will generate a test.cab file in your C:\Temp folder using the file list generated earlier.

Additional helpful reference: Microsoft Cabinet Reference. and makecab.exe details.