Sorry, you might misunderstood me.
I did not create Forge for this, since I knew it was already created but under approval (which is longer in ODC), and I knew from support chat that Multipart is scheduled for built/in support (like in O11) in Q4 2003.
I got helped by Principal for the multipart, but then me and my team allocated some times after some busy project, to learn about it.
You can open Multipart O11 Forge Plugin in O11 Service Studio, here's what I used (notice the app logo):

Create ODC Multipart Library Module in ODC Service Studio.
Then in O11 Service Studio, copy the structure Part, paste to your ODC Multipart plugin module

Next copy the O11's exception BadData, paste to your ODC Multipart plugin module

Then copy everything in red box from O11 to your ODC Multipart plugin module

You will notice many errors about BinaryConcat, which is not built-in action in ODC, and also not available yet in ODC Forge.

I will provide the BinaryConcat that you need to complete your ODC Multipart plugin Libray Module.
I will provide 2 zip files:
1. Harlin_BinaryConcat_Source Code
If your enterprise app do not trust pre-made runtime library, use this.
After extract to a folder, you review source code, then launch Windows PowerShell to execute file generate_upload_package.ps1 (provided you have installed DotNet Core)
2. Harlin_BinaryConcat.zip is pre-made runtime library
Just upload runtime library it in ODC Portal > External Logic > Create Library
After upload, you need to go back to External Logic, and click Review Content button, and publish it.
After publishing finished, then you can add dependency to HarlinBinaryConcat action.
Use HarlinBinaryConcat action to replace all BinaryConcat reference errors.
After all are replaced, then there should be no errors, and then you can publish your ODC Multipart Library and start to use it.
Hope it helps...