NOTICE!
Model will need customising to suit your purpose, and will not work “out-of-the-box” so to speak. My apologies.
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The STLs are listed below, as well as the Fusion 360 file.
I am uploading this because I haven't seen many models for the original Raspberry Pi 1B.
This is just meant to be a jumping off point to help people get started with making their own heatsink holder.
It is being submitted here because I figured someone could make use of it for their situation. The IC cut outs are in the correct place, as are the sections for other components.
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My Pi 1B is heavily modified, so this will likely need tweaking for your use case.
This is a heatsink holder I made for some heatsinks I salvaged from a mosfet and a stepper driver. The bottom flanges on the mosfet heatsink were fatigued off, then filed down with a small hand file. They are held in place using some paperclips that I bent around some screws, which are screwed in using M2.5 screws.
Mosfet Heatsink Stepper Heatsink

The author marked this model as their own original creation.