MWB H3 with composite roof leaking

[merry_fay]

I've got a former highways van with a composite roof. It still has the metal cross beams but the flat area of roof has been neatly removed and a composite sheet has been bolted over it.
Unfortunately it's leaking.
It's had 3M Schultz sprayed over the top of it and then painted over it again.
The leaking has improved but it's still coming through.
We can see/feel where the moisture is inside, but can't work out how it's getting in from outside.

Does anyone have any suggestions please as to what could help stop the leaks?
Maybe put a sheet of rubber over it? More layers of sealant? The most expensive option would be a pop top or less fancy version of a pop top but that's not currently within my budget...
Thankyou for your help.

[DanMK7uk]

Trying to picture it..

Fibreglass perhaps ?