How do I generate random numbers using a SeedableRng?

I have the following Rust code, the eventual goal of which is to generate a deterministic username given a public key:

use openssl::pkey::{PKey, Private};
use rand::{Rng, SeedableRng};
use std::convert::TryInto;

pub struct MyRng([u8; 32]);

impl SeedableRng for MyRng {
    type Seed = [u8; 32];

    fn from_seed(seed: Self::Seed) -> MyRng {
        MyRng(seed)
    }
}

fn generate_username(keypair: &PKey<Private>) {
    let public_bytes: Vec<u8> = keypair.public_key_to_der().unwrap();
    let public_bytes: [u8; 32] = public_bytes.try_into().unwrap();
    let seeded_rng: MyRng = SeedableRng::from_seed(public_bytes);

    let num = rand::thread_rng().gen_range(0..32);
}

Using rand::thread_rng() to create a ThreadRng allows me to create a random number without a seed. If I change the last line to seeded_rng.gen_range(0..32) then I am told a method is not found, and the traits Rng and RngCore are not implemented. However, I am unsure how to go about implementing these traits.

How can I generate random numbers using a seed of [u8; 32]? Also, are there any areas where my code could be improved idiomatically? (e.g. is it necessary for my to create my own struct?)