This is my current collection of collar stays for my dress/work shirts.

These are modelled from my actual collar stays that came with my work shirts which needed replacing as some broke over time and others went AWOL.

I suggest printing these in PETG as it is flexible yet stiff enough to do the job.
I tried printing in PLA, but they snapped in half way to easily.

The nice thing about these models is you can up/down size them in your slicer (x and y axis) to fit the length/width you need. I would keep the z axis (how thick they are) at 1mm as I found 0.5mm to be too thin and did not provide enough stability for your collar tip.
1mm seems to be the 'just right' spot for PETG to provide the strength yet flexibility needed to keep your collar tips straight.

I designed these in Fusion360 by taking a picture of my collar stays together, importing the picture and tracing then extruding them.

I may add more to the collection in the future but I find just resizing them as needed from these files works for basically all of my work shirts.