I just did catarac surgery on it and I carefully examined the thickness of the moldy material and decided to use that 3M double sided foam tape(roll) that is very common and white. It is used to put posters, etc. on the wall. I had cut the original clear tape at the lower edge of the lens all the way around so the tape stuck up as a border when the lens was removed. I placed the 3M tape around the outer most edge of the tube and some of the tape won't make contact at the outermost edge of ther lens but the inner area of the tape will. The tape might be a hair thicker but a little stretch as it is applied fixes that plus the small area of foam tape contact will compress. It is firm but pliable. I think it would do just what that gasket they have talked about from the 21FB's does. I think it is perfect for the job. I put it back together leaving the original tape around the tube and used scotch tape to seal the split in the tape around the lens. The metal braces mounted exactly where they were with lens securely in place. A dark grey or black tape would be nice but don't know of any yet. When installed you can see a bit of the tape if you look hard at angle between frame and lens. Basically it is perfect.
Finally, my CRT looks right! Now if only I can get the heater cathode shorts out! I'll give an update. I did not take a photo of my job because I have a million things apart around here and I felt I could not stop to photo plus my description should be good enough for this. I will post photos of other stuff soon.

There is a black double sided foam tape used to hold trim on cars. I think it is similiar in thickness to what you are using.

Did you say heater to cathode short? Put a brightener on it. The most common kind of brightener for these sets has a switch on top that allows you to run normal filament voltage, using the brightener only to isolate the heaters. This is a quick and dirty fix for a heater to cathode short.

John

I think the black 3M tape is high strength for emblems but I think 3M has a dark grey with average stick.

That brightner idea sounds good but I kept mess'n with rejuve and remove short process on the B&K 445 and currently the tube tests good with no shorts. I was very surprised. I spent alot of time with bad results til I had given up and tried one more time. I won't know if the tube has a good pic till I fix the chassis that needs caps. The 445 has had little use but a lttle moisture exposure while it looks great inside and out. The original tube looks new but the two electrolytics are original so I wonder what they do and if I have accurate test inspite of the caps.

It will take me awhile to fix the chassis because I am trying to get original type cans.