TreeBoxes ...or simple mounds of dirt over the root ball which after settling that will sufficiently cover the root ball.

I used to do a lot of tree planting for an Arborist @ the Triangle Nursery in Bellevue and the rule was to never dig into the clay at all when planting. This is because the basin in the clay will simply create a clay pot of sorts, resulting in a puddle and drown the rootball out and kill the tree. So you can plant with a minimal hole into the dirt and mound of good soil over the rootball no problemo. Stake the saplings for a year and you should be good to go.