+1 for
Mr T’s brown parcel tape suggestion.
I’ve used that on everything from a yoga mat…
To cereal box cardboard…
To modelling mesh…
If the spray adhesive doesn’t work, perhaps some duct tape might?
( Joined in circles with the sticky side out. )
If you are making a headliner, can you put the bodyshell (carefully/supported) on its roof?
That way, the weight of the fibreglass matting going on will help keep everything in place.
Good luck, Paul.
