Replica 21FBP22 gaskets?
 

Does anyone sell replica 21FBP22-style lens gaskets? My understanding is the 21FJP22 and 21FBP22 are electrically the same tube with the only difference being the FJP has a bonded lens and the FBP has some sort of rubber gasket which holds the lens. I'm planning to deal with the cataract on my 21FJP22 soon and presumably it would be much easier to just reattach the lens with a gasket instead of having to use double stick tape and/or silicone which has a lot more opportunity for error.

The original gasket on my tube deteriorated badly, getting soft and oily, practically liquified on the surface. I suspect any NOS gaskets would be the same, if any even exist.
I don't think Scotty had any for rebuilt tubes. Tape makes a lot more sense to me.

I note Doug Harland recommends re-using an old gasket after cleaning with soapy water and applying a rubber renewing product.
https://www.youtube.com/watch?v=VlWXNVJ5fN0

Now that I think about it, he may have reused mine AND applied tape to seal it or just hold it in place.

Unfortunately, you don't have an old one to reuse.
So, I hope you have luck in finding either an old one or a replica.

There's a lot of door and window gasket available in hardware stores...If I had to make a gasket quick I'd look at that...
Personally I prefer silicone* in the small hand tubes that don't require a caulk gun....you get much more control with the small ones and can lay down the minimal bead needed to seal it easily. (The kitchen and bathroom version of this) https://www.homedepot.com/p/GE-Advan...-3TG/100179996
Roundy CRTs usually have a solid half inch of unphosphored bulb edge and the cabinet mask usually cuts off a bit of the phosphor all around too... even when I'm not fully concentrating I don't think I've done worse than 2/3 of the unphosphored edge and unless you install it in the cabinet WAY off center you'll never see it even if you press your face to the glass and look at strange angles. Generally try to achieve a fairly even ~1/4" bead between the two.

*If you get the glass spotless you'll never have to open it to clean it again...gaskets may leak or their material degrade but silicone if kept out of the elements should last my lifetime. The one of the first CRTs I did that was caulked almost a decade ago still looks perfect.
You can open the silicone up if you have to...2 of the first 3 tubes I did had Electronic defects (one the cathodes died, the other was gassy) and a few years later I harvested the safety glass from those 2 with an exacto knife to reuse on good CRTs that had worse condition safety glass.

Keep it simple!
In the shop we used clear packing tape. Gave it a couple of layers. Worked like a champ.
If you have to remove the safety glass to clean that smudge, no problem.
Believe me, I had an occasional face that looked spotless until I fired up the set.
I was so glad when bonded tubes made their appearance in the aftermarket.

What does the cross section of the gasket look like? An L or a U? If you look hard enough, you can find generic gasket material like that.

Solid advice all around, I will go with silicone and tape.

I'm interested to know, did the gasket actually attach the glass to the CRT or is it just a spacer and it would have been taped from the factory? I was assuming the former but the drh4683 video seems to show otherwise, he just lifted it right off...

The gasket was just a spacer and a partial seal. Tape was not used on the gasketed CRTs (only the bonded ones). The gasket tubes were held on by the CRT cabinet mounting brackets.