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Cambodia no longer offers a transit visa and you will have to apply for a standard eVisa online, this will cost $36 (USD).
From the visa page the requirements are:
Passport validity of more than six months balance at the time of entry, a recent passport-size photo in digital format (JPEG or PNG format), a valid credit card (Visa/MasterCard).
Additional details that the form asks for are:
You won't be able to get a visa on arrival in Aranyaprathet as tourist visas are available on arrival at Phnom Penh or Siem Reap international airports only according to the UK GOV website.