Dog Wheel Chair Adjustable Bracket Prototype

I was looking for an adjustable no glue attachment method for the brackets from Erica Charbonneau's FIGO Rear Support Pet Wheelchair -https://www.thingiverse.com/thing:1397964

This is what I've come up with so far. As you can see, it cracked a little at the edge when I tightened down the bolt, so it look like it might be necessary to extend the bracket a bit so the bolt hole is not so close to the edge. Other than that, it holds really well. Using the existing bolt hole designed for the str