Turned out I needed a bit of metal at least 4mm thick to be able to clear the top cross brace of the frame.
Ended up cutting 3 small pieces of 1.5mm and messed around with them until I got something that kind of would work (most definitely not on par with Berna's work - still have a lot to learn before I could even think of building my own trucks)
Hard to see in the picture, but those 3 little pieces of aluminium took me the better part of today to get to that state. Learn while you go and fun at the same time. Will use M2x10 model bolts and nuts to bolt it all to the frame.
I had made the holes in the plates .5mm oversized to give me some wiggle space, but it looks like I will need bit more filing and sanding on the sides to straighten it all out a bit more.