Hi Pete!
I've used them a couple of times a long time ago. The reason it was a long time ago is because I can't say I was overly thrilled with them. On the one hand, I can see great potential in them. I think, as a concept, they're pretty neat. You peel them off the backing, dip them in soapy water, apply it to the canopy and then let it dry. Voila! Instant framework.
However, on the downside, you're not exactly applying decal-type material. The material is like a plastic sticker (for lack of a better term) and it will sit on top of the raised frame detail that is molded onto the clear plastic part. So if your clear plastic frame has any sort of detail on it, the Fast Frame will merely rest on top of it and it's sort of two-dimensional. I can't help but wonder if it would work out so much better if you actually sanded off the raised frame detail from the B-29 canopy and then apply the Fast Frames. I can't say for certain that would actually work, but I do wonder.
The last time I used them, I applied a Fast Frame onto my Accurate Miniatures P-51C. To its credit, it's been about 12 or 13 years and the frame has NOT lifted off. I'm kind of surprised by that.
So, in my opinion, there's the good and the bad of it. The B-29 has gobs of framing so I don't blame you for considering the Fast Frames. Personally, I squeeze a bunch of white glue into something like a plastic soda bottle cap. I add a drop of water (or two drops max), a small drop of liquid dishwashing soap and mix them all together. I then take a small paint brush and brush on a thick coat onto each window pane. After all the panes have been covered and the glue has dried, I then paint the framing. When the paint is dry, I go back with a wooden toothpick and pick off each rectangle of the glue mask.
One word of caution if you try this route though. Don't let the dried glue sit for more than a day or two before you start to paint. If left on for several days, the glue may start to shrink and the paint will bleed under. I usually mask at night and then paint the following day.
I hope this has helped!
Eric