The best method will be to help the tree grow some new roots above the current set of roots, that way the tree will have
enough sub surface structure to help hold it in place over the long haul.
It is a rather major undertaking but it can be done, you will need to first mark around the trunk approx. 3" wide x 4" tall rectangles, no cutting the bark just yet, simply make some marks equi-distant around the trunk with spray paint that is easy to see.
Next you need to either buy or make some rooting hormone so you can stimulate the tree to form the new roots where you have marked the trunk, Willow bark is a naturally occurring rooting hormone (the inner bark is where the active ingredients live).
You also will want to have a pile of soil for each tree you are going to force to form new roots on, this will end up piled around the trunk to cover the new
root spots you marked with the spray paint, the soil will be what is kept moist to encourage the new root growth.
Once you are all set it is time to make some cuts in the bark and apply the rooting hormone, cover with soil,
water till good and damp and start the monitoring for moisture content.
If you are going to also or instead stake the tree, you want at least three and preferably four lines around the trunk make sure the lines are well padded so they don't bite into the bark. You will be changing these out at least once a year and better would be beginning of spring, middle of summer, beginning of winter.
Hope that helps you solve your problem.