It's an old topic, but think it's worthwhile sharing my experience. I have just fitted secondary 4mm acrylic DG using self adhesive magnetic strip 19mm. I have no association with the companies mentioned. I bought the mag tape from https://www.first4magnets.com/tape-strip-c85/magflex-19mm-wide-flexible-magnetic-tape-premium-self-adhesive-p3526#ab_1-16|ps_2-1789 and the pre-cut acrylic from https://www.livsupplies.co.uk/product_list_poly.php?id=20. My windows are inward hoppers and as such i had to trim the stay so that it didn't protrude beyond the rectangular wooden frame. I found that the easiest way to do this was to saw through the angled bit and then heavily score the flat part. Then using two pairs of pliers the aluminium can be fatigued along the score line by bending back and forth until breakage occurred the finally smoothed with file. The next step was to fit the mag strip all round the edge of the acrylic . On the first unit I did, I mitred the corners but on subsequent units I left the joints square - easier to get a closer join. Once the mag tape had been stuck to the acrylic, I cut duplicate pieces and magnetically attached them to the strips previously stuck to the acrylic. NB the tape I used was self-mating and will only match if the tapes are in the correct polarity - if you use mitred corners, make sure it's the correct way round, if cutting square then it doesn't matter as you can simply reverse it.
Now i I made my first mistake doh. I attached the pre-cut duplicates to the window frame - big mistake. The problem is that with sel-mating tape the two pieces have to match perfectly for the magnets to stick together. Even a couple of millimetres out and the magnets won't stick, as I discovered. Luckily, I hadn't stuck the tape to the frame firmly and pulled the bottom and side strips off without losing too much of their adhesion. I then replaced the removed pieces onto the acrylic. I then positioned the sheet such that the top strips mated correctly and then simply pushed the sheet firmly round the edges into place. On subsequent units, I left the tapes on the acrylic, removed the glue protection on the top tape, positioned the sheet and pressed the top tape onto the frame and then removed the glue protection from the side and bottom tape and pushed all the perimeter firmly onto the frame. Perfect.
Total cost for 6 Windows around £150.
I have labelled the units so that they can go back onto the same window, although it shouldn't really matter if all the tapes have been fitted perfectly in the first place. Another point to note is to ensure your frames are clean and smooth before finally pressing the units in place.
Hope me this helps others to not repeat my mistakes.