Hi Mossiepilot,
I have used pvc electrical tape but in a different way - I have cut a length of the tape strip, then stuck that down to the cutting mat then primered the surface, then when dry, cut strips of it into 0.5mm strips (or thinner depending on the aircraft canopy - good scalpel blade required for this!) and applied the strips to the canopy where the frame marks appear, the stretchy tape takes on the contours well and can be pulled tight as well. Some of the 'cheaper' pvc tapes are a lot thinner and are ideal. I've found it best to give canopy a coat of Klear first and left to fully cure, then when strips are applied correctly I paint the strips in the finished colour with a long haired 0 (or 00) size brush.
I did this on a 1:72 B24 Liberator with good success on the front and rear gunners and pilots canopy (which is similar to the 'greenhouse' canopy of the Lancaster). If the tape is burnished down well the result is very realistic and gives a true framed canopy appearence. Finally I give a coat of
Revell clear varnish to the canopy, in between the strips which gives a very good high gloss glassy appearance and just nicely takes up any slight edge to the tape strips. The end result is a very realistic appearance to the model with a realistic looking frame. The beauty of it is if you don't like the finished appearance or it doesn't go well it can be all stripped off leaving a clean canopy. I have used this method on other smaller models with good affect. Hope this of some help. Forgot to add this method gives a nice clean edge as well!