so i just did some quick googling and even cheap plastics have a melting point of ~212° F. you might just be okay. i'd wait a week or so to monitor your heating and see how it goes, but a temp gun will help here. go ahead and try some googling for yourself so you can explain to your dad in your own words.
EDIT: so i just asked MY dad (he's an engineer) and he says this is definitely a fire hazard, because worst-case scenario is not worth the risk. it CAN present a fire hazard. definitely try the 75W. i'm very sorry for the confusing answers, i'm just trying to help!!!