I use Dubro flat hinges every time that I can. What special tools are needed?

Snap off a short section of hacksaw blade. The broken piece will be perfect for enlarging those slots. The teeth will cut out the right thickness and pull the balsa dust out at the same time. Take a little care with each slot. Before you start, look to see if the slot is actually centered and straight. It's amazing how many aren't perfectly centered or straight. With the hacksaw blade, when you're widening, you can work the slot to be straighter and better centered.

That's it for me.

I do have a Slot Machine and it's good, but won't straighten or center nearly as well as that no-cost tool.