The absence of any flour or custard powder means that the curd can’t be boiled or you will end up with scrambled eggs. To play safe, warm the orange curd in a bowl on top of a pan with simmering water (au bain marie).
At 65°C, the egg starts to bind the curd. At 75°C, the egg are cooked enough to be safe to eat.
The less butter you add, the thinner the curd will be.
Use a different fruit juice like lemon, lime, mango or raspberries.