yes...3Mbrand ..... it is reliably very good. It is two sided and often used to hold that plastic trim on car panels
and quite a few other applications. Not limited to a marine
source. The stuff I used was only 1.6 mm thick but I faired and edge all around the inside to even out the waves in the steel
work so where the sealant
was, was between 3 and 6+mm. The tape has serious grunt. A mis-alignment in window instal is no fun. With no adhesive
action by the sealant, the tape should be more than sufficent to hold the polycarb. I understand it is availabe in a variety of thickness's.
Your 5 mm dam would make things easier on your sealant and allow for much larger plastic. The size I was reporting on were about 1 foot (300mm) square kinda sorta. The silicone I specified is intended for those big glass and steel
buildings with tinted glass panels
of very large size. 2X3 metres for example, and has an extreme amount of 'give'. One of the things I liked about the system was that it was rather low profile relitively speaking.
that sounds like the stuff alright!