The big issue was less about fitting the plastic back together, but getting the Trinitron inside to stay put at the appropriate height. In Apple's original design, the bezel bore all the weight of the tube via four mounting screws at the front- which are now a fine powder and a faint memory. The repair job, while "passable" cosmetically, is not ever going to be able to bear that sort of load, so a new method was called for.
I knew I wanted to use foam for this part. Foam is easy to cut, shape, and form to fit all sorts of things, and with a neat "shelf" situated just under the monitor and above the disk drives that was able to bear the weight, it should be easy to get something in there to do the job. Thankfully, it was.
While this did succeed in elevating the tube up to a more appropriate height, a new problem developed- the rear of the tube lacked support, and the neck board for the Trinitron was touching the analog board. Probably not good! My initial solution was a two-part foam riser made out of cushion foam, a type of compressible packing foam, and that looked like this:
Immediately this had a number of flaws, but did support the tube long enough for me to get some better measurements. The key issues are that the foam here is covering components that can get quite hot (and in turn melt/burn the foam), and that it is not strong enough to bear that sort of weight, and would quickly compress until the neck board was again resting on the chassis. So, a new solution was needed.
This was my next and current solution- a piece of foam curved to fit the monitor secured via the same glue to the foam in front of it- though the weight of the tube does virtually all of the work keeping it in place. This is more a "proof of concept" so I can advance to testing the electronics- this will need a more refined and robust fix before this machine will get my all-clear. The last thing I want is for that tube to come flying out at someone unsuspecting, or to explode in a shower of sparks if it cooks itself on the electronics. I'm considering something more robust for this part, but I'm rather fond of the insulating properties of foam and the ease of shaping it to fit what I need. This will require more consideration I think.