Hi @harrison.friedes ,
We can do but it depends upon various factors, number of mesh facets and with multiple tries.
Working directly from the mesh is a good approach when you want to preserve the original fit and clearances.
Here’s how I handled it:
1. Insert the mesh and create face groups
After bringing the STL into Fusion, I used Edit Feature(Direct Edit), Then used Create Face Groups. This helps isolate the “L”-shaped region so it can be removed cleanly instead of trying to edit triangle-by-triangle.


2. Delete the “L”-shaped hole region
With the face groups in place, I deleted the faces that form the “L” shape. This closes up the opening in the mesh and prepares it for conversion.
You may need to repeat this step several times to delete the fillet in the L shape

3. Convert Mesh → BRep
Once the mesh was cleaned up, Finish 'Direct Editing'
I used Mesh → Modify → Convert Mesh to turn it into a solid BRep body. After conversion, the model becomes far easier to edit and modify without dealing with all the small mesh triangles.

4. Now you can cut any new shape or letter
With the body now in BRep form, you can sketch whatever symbol or letter you want and use standard solid modeling tools (Extrude, Cut, Emboss, etc.) to create the new design.
I’ve attached an image and the .f3d file showing the result so you can see exactly what the workflow looks like.