Rex,
 The windfall 1/2" acrylic is certainly a way to go since you have the 
 means to make clean enough cuts for a pressure tight case. You
 might have noticed the common 12" x 24" dimensions that I am
 using so as to be able to use pre-cut sizes from McMaster-Carr.

 For the DB-25 connector feedthrough: I looked at a gender-mender
 and decided that the pins were not sealed. So I built one into
 the feedthrough by using a male solder-cup connector on the bottom
 and a long machine-pin PC board female on top. I removed the shell
 halves (drill the rivets at the screw holes) from the top connector 
 so I could separate the halves of the insert that holds the pins. 
 THe inside (long pin side) of the shell and the insert are discarded. 
 Then I cut/filed the upper half (connector pin side)of the insert so 
 it would fit through the the hole in the 1/2" polycarbonate panel, 
 which was about the standard DB-25 panel opening. 

 Then I soldered the long pins into the solder cups with the "outside"
 half of the insert in place. The insert can then be removed and
 the assembly was then pushed up through the cutout from the bottom 
 and the opening filled with RTV sealant.
 Then the upper half of the insert and the outer half of the metal
 frame are installed with long 4-40 screws all the way through the
 frame of the lower connector.

 The seismometer is already equipped with a ribbon cable and a
 DB-25 connector. The 25 conductors are more that what is needed, but
 the convenience of the color-coded cable makes it easy to work
 with (I use it for much of the internal wiring of the multi-period
 electronics box), and the "extra" leads can all be tied together and
 used for ground.

 The 25-conductor ribbon cable has a right-angle connector on it
 (the clamp-together-without-a-shell-but-with-a-strain-relief kind),
 so minimum clearance under the case is needed. I use the 1 1/4" long
 brass torpedo-like fishing weights drilled/tapped 1/4-28 as feet
 for the aluminum support plate, and the shorter ones for the 
 seismometer itself to rest on the support plate.

 By the way, it helps to have the motor-driven mechanical zeroing
 operating before one seals the seis into a pressure case. I think
 I have described it before.

 Regards,
 Sean-Thomas

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