Heater Delete Cover Cement

[blackss69]

I am looking for information on the 3899878 Cement-Plastic Seam (item 67a on C48 Heater Delete in AIM).  There is a gap to seal.  Does anyone know the composition of this or if a good substitute is available?  I guess I could use the 3M ribbon black sealer but would like to know about what is really correct.

Thanks