Remapping modifier keys and others via XML .keylayout file

I have a video editing keyboard that requires some keys to be remapped to work correctly on my mac. Specifically I need to remap the modifier keys on the keyboard. The first thing I need to do is remap key 50 to be command instead of ~. I am trying to do this via a custom XML .keylayout file. I have read the documentation on .keylayout files and can't make sense of what it says to do. Also, I've tried many of the keymapping tools that people have recommended and they don't work for remapping modifier keys.

Any help with this would be much appreciated. Thanks.