Tick the box that says 'Business' on the visa form (instead of 'Tourist'), pay $25 instead of $20, you get a 30 day business visa. Go to any one of dozens of visa agents in Phnom Penh, pay the going rate (about $260-280), and ask for a one-year extension of business visa. Takes a couple days, it will automatically be multiple-entry, and the process is hassle-free. Repeat as needed. No need to exit Cambodia, no need to report every 90 days, no need to ever SEE Immigration. Ever.