Lateral support for new basement wall?

I want to build a bathroom in the basement. It will be in an existing corner, so visualize two new 8' walls to make a square in the corner of the basement. The floor joists above are open web trusses and are to be supported only at the existing load bearing walls. So I am wondering about how to provide lateral support at the top of the two new walls. I could run some pieces of 2x across the bottom of adjacent floor joists and then drill a hole through the wall top plate and into the piece of 2x then drop a bolt through, leaving space between the plate and the 2x so the joist would not be resting on the plate. Or, I could run some 2x from the existing basement wall to the top of the new wall and fasten to the plate. Any thoughts?
Thanks!