taking a warp out of a piece of phenolic laminate

I have a piece of phenolic linen laminate that I'm hoping to use as either the top or bottom containment plate for EML2 (railgun #2).  But it's got a longitudinal warp in it.  That is, it's curved from end to end, but not across the width.  The material is 36.25" long, 4.75" wide, and 1" thick.

It's the thickness I'm worried about.  I could mill the thing flat, but then it wouldn't be 1" thick.  In fact, I need to mill the bore portion flat anyway, and I'd like to remove as little material as possible.

While it's possible that I could pull the warp out just by bolting it to the other (presumably straight) parts; I wouldn't count on that, and in any case, I need to be able to verify that the bore is straight and flat in both axes with the launcher disassembled. I could clamp it to the more-or-less-flat table of the mill, mill the required features, and then let it spring back, relying as I said earlier on the clamping bolts of the launcher assembly to pull that plate straight.  Of course, it might just pull the other plate and the side walls and everything else slightly out of true too.

For now, it is clamped to my surface plate, which is too small to hold all of it even diagonally.  I might have to borrow the surface plate at work over a weekend and see if I can apply heat to it (with a bunch of heat lamps, I suppose) while it is clamped flat.  I'm assuming heat caused the warpage in the first place.  Of course, I don't know whether phenolic warps TOWARD or AWAY from the heat source.  In the case of wood, it's easy, because it's a drying effect.  In the case of a thermoset resin, I really don't know.

I've been trying to find out, but no luck so far.