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Jason,

   First off plugging the hole will be a personal quest as no one with a
business in in a position to break the law or risk the liability by helping
you plug these things openly.  
Pun not intended - but pretty good huh!

I can't imagine not having enough room to stack disks under a plug but then
you are there looking at it and I have not seen a Sherwood manifold set up
that closely.

If disks won't fit under your plug you will have to fill the plug with solder
as George suggests and use a regular disk.   This will not really have the
same effect as a nice thick "no pop" unless you do it exactly right so that
the surface meeting the plug is dead flat and the solder joint is excellent.
  For example if the disk leaks because of a tiny puncture or tear gas will
slowly leak past any cold or poor solder joint between the solder and the
plug.   Also if a poor solder joint breaks loose the disk will bulge into the
hole and simply push the solder plug out of the way if there is room for it
to move so you will have to make sure the solder plug completely fills the
cavity in the plug or you might actually be diving with a regular disk with a
plug of loose solder over it without realizing it. 

I am not sure what to suggest in light of your valve threads being too short
to stack disks.  You are right to not make any changes that you are leary of
but this situation just doesn't sound right.   A no pop only needs to be
about 3 time the thickness of a 3000 psi rated disk (.03" to .06" should be
plenty) and one should fit under you plug.  Perhaps there is a slight curve
to them that makes your stacked disks stand up a little higher than when
compressed.  

My suggestion is to find an experienced tech diver with mechanical aptitude
in your area who will help you with this.   Don't trust your local
recreational dive shop people with this stuff.   
A machine shop should be able to cut or punch out some plugs for you but it
will be up to you to make sure they are right.   You might be able to contact
some of the dive shops in the cave diving region of Florida and order a few
no pops - again liability might present a problem in some cases but a phone
call to the right person can work wonders if handled right (try Spring
Systems Dive Center).

If you ultimately just put a regular bolt in the hole some observant guy
filling your tanks is going to notice and refuse to fill them so it will have
to look right no matter what you do.  

This hole is a serious weak spot in your life support system - possibly worse
than the O' rings in a manifold.   It always represents a nerve racking
unknown unless you treat it like you should all your equipment - check it out
yourself and get the situation under YOUR control rather than than of some
engineer or some tired machine operator in a factory in Tiwan with a hot
date.    

Hope this helps some.  Maybe some others on the list can add to this. 

Chuck

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