Extend L-Visa

I would like to extend my L for another 30 days. I came here by myself and have a registration form from the local police station. They told me I need a bank certificate with at least 3000 US$. Does that need to be on a chinese bank account? I only have a print form from my online home bank (but that looks like self typed without any stamp or logo) I also don't have any invitation letter as I came here to study but used my tourist L for that as the duration was the same.... I'm planning to leave China by train to HK, but it's too soon to buy a trainticket yet so I can't prove my 'leave'....
Thank you for your help!!