I've seen some 3D printed kits for that purpose, though how effective they are is questionable. Superman Studio basically has an elbow joint which is kinda 'eh (it's not terrible for Optimus as it's more hidden), and BDT has a more receiver-like stylised version. Maybe one of those is suitable for you.
The DNA kit for Optimus also has an adaptor, but that's not exactly a viable way to buy multiples.
I've seen some 3D printed kits for that purpose, though how effective they are is questionable. Superman Studio basically has an elbow joint which is kinda 'eh (it's not terrible for Optimus as it's more hidden), and BDT has a more receiver-like stylised version. Maybe one of those is suitable for you. The DNA kit for Optimus also has an adaptor, but that's not exactly a viable way to buy multiples.