The slits are not made of particles, they are slits, so the absence of particles. You could make a double slit experiment by shining a laser on a single strand

The slits are not made of particles, they are slits, so the absence of particles. You could make a double slit experiment by shining a laser on a single strand of hair, this would create two wide slits on either side. Or draw black sharpie on some glass, and scratching two openings on it with a needle. Since effects are clearer when it matches wavelength, you can also buy pre-made ones if you don’t feel like making it yourself