Instead of ranting more, I'm gonna go through how I made most of this, because finding anything related to making object collision from scratch with Nifskope is like searching for fucking bigfoots older brother.

How it was made:

• First thing I did was merge the plane pieces together in Blender 3.1 (because I cannot navigate 2.49's UI yet) then I merged by distance and limit dissolved it.
• Then I export the mesh as a NIF file.
• I followed this video https://www.youtube.com/watch?v=k5LsygGOCpk&list=PLQCtyYvLm2OPWTfDilf-6Y2Pxucx92IGk&index=4 to turn the merged mesh into collision
• I opened both the original nif and the merged nif, renamed the merge objects block to the original's block names, then copied the original nif's branches over the merge nif's branches
• Applied most of the spells in Nifskope (Update all tangent spaces, update all MOPP code, ect)
• But...
  

There was no collision in game. There was not even collision block in the GECK when inspecting the model, so I asked about on some Discords, then asked in the Nexus forums for help regarding the issue. After a few hours, Pixelhate contacted me in Webb's Discords, and fixed the plane's collision by adding a sub shape that was missing in hkPackedNiTriStripsData and optimised the collision mesh as an added bonus.

Thanks to them, this mod is finished, so thank you again.

Tools Used in Order: 

B.A.E - Extracted the game's assets from the BSAs with this.

Blender 3.1 - Merged the plane's 3 meshes into 1.

Blender NifTools - Adds Blender support for Bethesda file types such as NIF.

Nifskope 1.1.3 - Converted the merge plane mesh into a collision mesh, more detail in "How it was made".

Blender 2.49b - Used to optimise the collision mesh