I'm kinda' late to this discussion, but I just had an idea for a device
that would excel in evenly-applied-force. That would be an air bag,
fitted in a box. A reverse of the vacuum air bag, which is limited to
15 psi. I don't know how much more that 21 psi you'd want, but a
bag-in-a-box could do much more than that.

For a bag, how about the bladder that's used in the storage tank for
well water, or in the expansion tank of hot water heating system. The
bladder itself is available as a replacement part.

Bob