By "sheathed and glassed" it sounds like you're planning on sheeting it with balsa and then light glass cloth and finishing resin? If so then the sheeting and glass will provide all the torsional strength you need and then some. So don't bother with the diagonals. I'll also suggest that adding that sheeting and glass finish will result in wings that are far over 10 oz total... especially after painting.

But the good news is that you've got lots of wing area so the model should still fly just fine.

PS: wrong forum. This is about structure and not aerodynamics. Moved....