Actually, if no HairDryer ... and you're force to use lighter...

i'll give u a idea,

Go get a aluminium wrap  <-- if dont have? see below

Go buy Mentos, take the silver color paper outside <-- if you dont like sweet? see below

Smooker? Get a hard pack, and when u open the package, you'll able to pull a silver color paper  <-- if you dont smooke? pls consider the solution above..

wrap the heatShrink, and try to burn it... it will not leave the black smooky stain on the rubber, no worry 

give a try 
* i always use this way to seal plastic..