Hello Dougone and Welcome to the Forum:
Go to the attic access and look in; if you have trusses running perpendicular to the wall in question, you won't have to support the opening at all. Trusses are designed for full span support; all you would need to do is repair the vacancy left by the wall.
On the other hand; if you have joists and rafters and the joists are spliced over the wall, then yes you do need to support both sides. I would put in a 2"X4" for each joist, I would also use a 2"X6" or 2"X8" on the ceiling and floor to distribute the load evenly.
The beam sounds fine if it is needed; it could be made much stronger by sandwiching a steel plate rather than the plywood. The plywood core can be glued and nailed in place and doesn't cost near as much as steel, and the steel would have to have lots of bolts through it to make it work. Let us know how it turns out, maybe some pictures too.