Personally, I don't think you need to fake anything. Fake cumshots are common when models sell solo content. They can be really fun. Turn the problem into an opportunity. Sell the fake cumshot for $X and sell the real cumshot for 10 times $X. That way you are being totally honest and transparent, and trust me you will get plenty of takers on the fake shots. The worst thing you can ever do is lie to people.
What is more frustrating is that no manufacturer makes good gear for this. So you pretty much have to do some engineering on a higher-quality dildo, come up with a way to pressurize a stream, and find a way to make the whole thing look real. How many thousands of dildo designs are being sold online, yet not one vendor does even a half-decent job of handling fake orgasms. I have seen a couple of toys with squirt options, but they are cheap, even pathetic, designs. It's an opportunity for a sex toy vendor.