We have a bunch of photogates in our science lab, along with some nice carts by Vernier. I know Vernier has a cart flag for sale, and it is priced affordably, but I thought it might be nice to make a 3D printed version.

The design is meant to be flipped for different levels of accuracy. The single bar works great for just measuring velocity, but would not yield an acceleration. The multi bars are great for measuring acceleration, but make sure you specify the length in software.

1 bar: 50 cm
6 bar: 10 cm

3 bar: 25 cm
5 bar: 12.5 cm

I used a light color (yellow) with 20% infill and it was sufficient to block the sensor on the photogates.

I am not affiliated with Vernier in any way, just a science teacher who really likes their technology.