how to get consistent force inside a cure oven?

I need to make a fixture that applies around 20lb of force to squeeze two rubber pads together. Right now I turn a thread to what I think is the right force, but it is hard to know. One thought is a torque wrench, but not sure how consistent it would be since the torque is rather low. Another is to turn threads like I've been doing but have it stop at a certain point everytime and let spring pressure hold the force, but the oven temp is about 350 and I think the springs would weaken? I can also have something that sets the force outside the oven (same concept as a torque wrench), but I don't know what it would be. any ideas?

1] Be aware that creep in the rubber will tend to dissipate clamping force.
2] YOu can get high temp springs, though whether 350F or 350C will make a difference.
3] THere are incolloy springs that retain springiness at red heat
4] Belleville washers are springs, but look like washers bent into a somewhat conical shape. Add those to your threaded clamp to maintain force, though they don't have much travel. You can put them in series, though.
5] How about just putting a slab of metal on top of it ? Or a brick ? "Crude" and "primitive" are such ugly words; I like "elegant" and "simple" much better....

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