Thanks for the help here. As an update, I used a dremel, cut the channel in question to fit and all went well. I initially continued to have some issues with rounds feeding, but only when a single round was in the magazine. However, I used a sanding bit, and took a small amount in order to lower the entire piece (took material from the opposite end of the channel), and now all is well.

I think I'll have a happy 10-year old Christmas morning.