Like an idiot I had some walnut plywood cut and didn't consider the
grain direction on it so I'll need to apply some veneer over some
pieces.

Haven't used veneer in a while and when I did it was self adhesive
stuff I believe.

But on material without backing, what is the absolute best way to go?

Contact cements are hit and miss -- there are some good professional
ones though.

I have some 3M hi strength 90 spray (damn good) (but edge cleanup?)

Also have some 3M 468 transfer tape at my disposal I could use.

I know with the 3M 90 it will stay put but concerned about cleaning
the excess adhesive.