Thanks for the input xenologer! I'm fully aware of all the problems that come with a different rim size. I had 26" wheels before but since I couldn't find narrow 26" tires and I like the look of 28" rims anyway, I thought I'd give it a try despite the fact that it isn't the perfect setup.
As I'm on my phone and at school at the moment, so I can't express my idea any further.

The images from my last post work fine for me.

Now here's my idea to drill a hole into the already existing gap (which shouldn't make the overall structure much weaker).