express visa for Cambodia? (Ho Chi Minh City)

Hi all,
is there any posibility to get a visa for Cambodia within 1 day? What if I can't wait 3 working days?
If I need to go just across the border and return, how long does this take?
As far as I've seen, at Bavet the border gate is open from 8am to 8pm.
So, would it be possible to start with a bus from Pham Ngu Lao at 6am get an express visa (on arrival or so?), cross and recross the border at Bavet and then arrive back to Pham Ngu Lao at about noon?

Is there any bus company providing this ultra short trip?

Thanks in advance for your answers!

Best regards,
Jochen