It's not like an AK.
An AK has a rotating bolt with tabs that locks into ears in the trunnion. So most of the pressure from the round firing is put on the trunnion and bolt held.
The RPD has locking flaps on the carrier that move outward into grooves on the inside of the receiver, these flaps are behind the bolt head a few inches. So the pressure is further back in the receiver.
Do you have any parts of the RPD receiver to look at?
It still might be possible to make a receiver from something else other then a chunk of milled steel.
But even using heavy sheet steel or even tubing might be possible, but some machining would still be needed, and I think you would still need some parts of the original receiver (like the front and maybe the rear sections?)
But I get what your thinking, and it would be neat to see one built lighter, and the receiver is the bulk of that weight.
If you want more info I can look for pics I took or take some and hope they can help you think of something.
OH I just thought of this. In the "AK build it yourself" area is the "MG47" thread, now it mainly uses AK parts but it could help give you ideas.
Here, there's two threads, and it will take a while to go through them.