Everything is looking great! That's a metric crap ton of work to slice and dice this kit. It has virtually no joint mobility OOB. Keep it up!
For your slots, you could drill them out. Drill a hole at one end, move the part slightly left or right (depending on what end you start the hole, then drill another hole. Repeat until you reach the other end of the slot. Just be sure to drill holes slightly smaller than the desired slot width. After the holes are drilled, clean up the slot with a flat file.
Do you have a drill press? That would make the job a little easier.