Sealant for pop top vents

[petethefeet]

Hi all,
I have just removed a sheet of perspex that was covering the hole in the pop top as I want to fit an opening vent in place.
When I removed the perspex sheet there was some very sticky gloop holding and sealing it in place, it was like a runnier stickier version of blue tack similar to the stuff you use to bed the glass onto in a ali greenhouse, it looked like it worked well as it didn't not appear to dry out and so it flexed with the roof.
What is the best stuff to use to put a new bit of perspex with the vent in back onto the fiberglass roof??

Thanks

[petethefeet]

Well,
I have been Googling, my sealant of choice is normally Sikaflex so after looking at the one I have in the garage I have found it is not suitable for use with plastics that are prone to stress cracking (perspex) so looked a bit deeper and found Sikaflex 295 UV that is intended for bonding plastic boat windows in to fiberglass.
One tube on order, a bit expensive but I don't want to do it again in a hurry.