Ok you also need to decide reciprocating or non-reciprocating.
The FM 9mm upper w/ the forward positioned side charging works because it doesn't use a gas tube, so there is a rod that runs where the gas tube would normally be. Has a small spring to push it back forward, and it simply presses on the face of what would normally be the gas key.
Side receiver set ups can be two ways - reciprocating and non-reciprocating. For the love of god, why you'd want reciprocating support side on a semi auto (SCAR style... appropriately named) I don't know. For a manually operated gun like a Solo 300 or similar ? Youbetcha.
But yes, for reciprocating you simply need to drill a hole in the bolt to put a handle and cut a notch in the upper for the handle to run back and forth in.
For non-reciprocating (the way it should be god bless the FN-FAL and D. Seive ) you need a channel to hold something that can push on the bolt/bolt carrier but not interfere with the gas tube/gas key, etc. Don't need a return spring - the bolt will send it forward but not pull it back, or you can manually slide it forward. The way the Gibbz patent receivers do it (Gibbs, Spartan, NFA) is they use a special bolt cam pin with a extra ledge sticking up from it, this gives that side charging handle a place to push on that is both strong enough and doesn't interfere with gas tube/key. Other than the special cam pin the BCG is a standard milspec style BCG.
You MAY be able to do something like the HK/G3 cocking tube and push a rod against the same type of bolt cam pin that the Gibbz style use. You'd need a forearm with a rail in the right place (or missing one) to attach it along and to drill the needed hole in the face of the upper.
One member here in the past few months posted details on how he attached a side charging handle unit from a FN-FAL to a standard AR upper. If you are a McBendy/McGrindy type, that would be where I'd start.
Personally I just bought a Spartan from Joe Bob's. Best $250 I've spent. That and the $400 on a FM 8.5" 9mm forward side charging set up :)
If you need pics lemme know - wife's asleep and the beep beep of the safe opening wakes her up. She's a redheaded Puerto Rican, and I'd like to live through the night please.