Writing my own name on cheques

I have a cheque made out to my first name only. There is a tiny bit of room (maybe half an inch) at the end of the line where I can try to write my last name (which is fairly long). The handwriting clearly won't match.

I bank using PC Financial, so this will be deposited into an ATM.

I really don't want to talk to my aunt again. Should I try to write my own last name in, or will the bank deny the deposit?

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