Having rebuilt several MGA windscreens, I have a simple trick and one casual observation that may be unique and may help.

The first is a very simple tool to assist assembling the corner brackets. I call it a jackscrew. Pick up a #10-32 screw at least 1-inch long (longer the better), and screw on a #10-32 hex nut half way up the threads.Install this jackscrew through the perimeter frame into one of the corner brackets, threaded in just enough to make it hang on. This should not need to go through or past the far side of the corner bracket, so could not touch the glass.

Then run the hex nut down the jackscrew against the outer frame, and snug up the nut to pull the corner joint together. You can also use a scribe or small flat nose punch or a very small Phillips screwdriver, poked through the adjacent screw hole to nudge the two pieces into alignment while tightening down the hex nut to hold things in place. Once it is secured, remove the alignment punch and install one of the original 2BA flat head screws. Then you can remove the jackscrew and install the second 2BA screw. Repeat where needed to easily pull the corner joints together and install the screws.

For final assembly, lube the rubber packing strip with soapy water, wrap it around the edge of the glass, and press the rail over the rubber packing. Use clamps or screws or extra hands where needed to hold the frame parts in place while installing screws into the corner brackets. And now the casual observation.

-------------
For decades I had always used Moss Motors rubber packing parts. The glass should be nominally 1/4-inch +/-0.010 thick. I have always used Triplex glass,which is always the correct thickness. When Moss supplied thinner class (not Triplex) they also "learned" to supply matching thicker rubber. For thicker glass you need thinner rubber. Recently they have been supplying relatively soft foam rubber, which may be more forgiving of mismatch when the packing thickness is not perfectly matched to the glass thickness. For the most part it works, but not always as intended.

If the rubber is just a wee bit too thick, then assembly can be a "fight to the finish". Many of us have had great struggle getting these things together. I have personally had two cases of glass in my MGA cracking while driving some time after installation. A glass installed in 1998 lasted 16 years, 207,000 miles, and was replaced in 2016 due to long travel abrasive pitting of the front surface.

The first replacement in 2016 lasted 13 months, 38,000 miles and cracked in high heat of mid summer full sun in California in summer 2017. I wrote that one off as an odd fluke of thermal expansion and eventually installed another glass. --- Next one installed early 2019 lasted 5 days, 353 miles before it cracked, and I was so PO'd that I refused to replace it for a while. Then we had that Covid thing to delay things a while longer.

Mind you, none of these cracks originated at the screw locations. All of them originated at the frame mid way between screw locations, so it was not the screws causing the problems. I was beginning to think the frame rails might have been twisted (but turned out not to be the case).

When I finally decided to replace it again in early 2022, the glass and packing rubber came from Scarborough Faire (because Moss was out of stock for the glass). This one was from Fuyao in China, but good perimeter profile and spot on for 1/4-inch thickness. This time I left the posts and grab handles in the body (not to remove interior panels), removing 4 screws each side to remove the 4-piece frame with glass (very quick). I only removed four small screws from each end securing the side frame rails to the corner brackets. Having been apart recently, it came apart easily and I reused all of the screws.

More significant, the new packing rubber was solid rubber strip, not foam rubber, and it was correct thickness to fill the space nicely without binding. Single handed (no helper) I had the 4-piece frame apart and reassembled in 30 minutes, and the whole job start to finish was about an hour. Today (May 10. 2010) 13 months, 20,765 miles on, all is well with the world, and I expect it will not crack again. My recommendation is to get the correct thickness rubber packing to make life much easier.