Clamping question

[JAZZCNC]

I'm guessing, drill the holes and tap the threads.....Then bolt some spoil board onto the face...clamp the spoil board and machine the part from the opposite side ??

Any better suggestions are welcome

This will be your best bet.
Unless you have excess material thickness which can be clamped then flipped and surfaced away then it's very difficult.
Really only other way other than sticking it down is to do it in 2 halfs and change clamp position which is a pain.