front, end cap

[George]

I am presently repairing my 76' argosy 24' M.H.
I have a lot of hairline cracks/fractures in the OUTSIDE front end cap.
(the dome over the operator/passenger compartment OUTSIDE.)

I'm told this cap is aluimunum or galvnized metal well mine is NOT, it is a plastic of some sort, not fiberglass either.
I want to fill the cracks to prevent leaks as one would do to a fiberglass boat. BUT if I use the wrong stuff I will probably melt it .
Not the results that I have in mind
It appears the p.o.('S) ? have made some repairs (possibly low clearance problem) to the cap as it has some, what I would call, body filler(bondo?) applied. I would like to do it correctly the first time .
I should mention that I have a cap that appears slightly different to those that I have seen in the photos on this sight. mine did start it's life as a shuttle bus.Possibly that explaines the different cap. H E L P !!