Removing cup on thin stock w/o Jointer

Doing last year's Popular Woodworking - Fallingwater Inspired Table Lamp project. The shades are 3 1/2x16" walnut that's only 1/4" thick. After resawing, the boards cup slightly. Enough that I'm worried about the thin glue miter being able to pull them in and hold the joint. Stock was straight before the resaw (internal tension?). Stock has been in the shop for weeks, so I'd guess it had been acclimated.
There's not really anyway to support/pull the shade sides in the from back - I think it would interfere with the bulb. Don't think I can use the jointer - the shades are already thin.
Suggestions? Thank you...
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