In reply to the certificate question, most places shopuld be able to give you one if you ask. alot of trial flights are bought through 'experience' comapnies, such as Acorne and Red Letter Days, that have a presentation pack with one included for your instructor to complete.
As for the DIY logbook entries, or the so-called 'parker pen pilots'; in reality nothing is there to stop you except for your integrity. The aviation world is quite a small community and if you turn up for a job with 1000 hours having only just completed your CPL a few months before, your bound to meet up with someone who knows your past, and can deduce that your hours aren't kosher. If you're involved in an incident/accident (which by being a less experienced pilot than you claim to be may be more likely to happen) you may well face criminal charges along the lines of fraud and deception etc.