Your diagram is how I meant. You ideally want to make sure the frame sits such that there is no stress on it - e.g. in your drawing there are 6 feet and imagine the middle four were unscrewed so the frame isn't supported. The frame would then sag in the middle. If you poured the epoxy in this state, you would still get a nice flat surface, but if you change how the frame is mounted (e.g. adjust the feet, move it, add more feet etc), the frame will distort slightly and cause the epoxy to no longer be flat.
We're not talking much here - clearly the frame is quite rigid so wont flex much, but it's still something fairly easy to take account of which may help.