The brackets are made of mild
The brackets are made of mild steel. If you can find a vice and rubber hammer you can quite easily try bending it to the shape you need. I had to do some bending for my conversion to work.
If the innermost hole is the best fit and the upper arms rest on the fork properly in this position, by how much does the bracket bend outward? If the angle is not too bad then you can try bending it slightly to the parallel position you need.
Another option is to try fitting the brackets on the outside (non-wheel side) of the fork dropout. You will then have a very wide wheel dropout spacing, so your axle will need to be long enough. Your bracket upper arms (that rest on the fork arms) will also be out of position but these can be bent inward to rest on the fork. This I did on my conversion fork (image below). But my fork is 75mm - not the usual 100mm, so the final dropout width (and necessary axle length) came out alright.