How to generate a weak (e.g. 128-bit) RSA key?

I would like to do some testing with very weak RSA keys, like 128-bit or 256-bit to practice a little bit. But OpenSSL denies the creation of such weak keys with

$ openssl genrsa -out test.key 128
Error setting RSA length
40C7C155167F0000:error:1C8000AB:Provider routines:rsa_gen_set_params:key size too small:../providers/implementations/keymgmt/rsa_kmgmt.c:514:

I'm completely aware that such a key is about as secure as no encryption at all, because it can be factored in milliseconds, which is the whole point of the thing.

Aside from manually piecing together a weak key in a hex editor, how would you do such a thing?

Newer versions of OpenSSL do not support genrsa numbits below 512.

>The default is 2048 and values less than 512 are not allowed.

I had no issue using openssl genrsa -out rsa.key 128 with LibreSSL 3.3.6

Here's some Python code that will generate keys; modify it as needed.

WARNING: These keys should never be used for anything.

import rsa

pubkey,privkey = rsa.newkeys(128)
print(pubkey.save_pkcs1('PEM').decode('UTF-8'))
print(privkey.save_pkcs1('PEM').decode('UTF-8'))

pubkey,privkey = rsa.newkeys(256)
print(pubkey.save_pkcs1('PEM').decode('UTF-8'))
print(privkey.save_pkcs1('PEM').decode('UTF-8'))

Output:

> -----BEGIN RSA PUBLIC KEY----- MBgCEQCl/IGtHk0lD0CvdwOKbFtzAgMBAAE=
> -----END RSA PUBLIC KEY-----
>
> -----BEGIN RSA PRIVATE KEY----- MGICAQACEQCl/IGtHk0lD0CvdwOKbFtzAgMBAAECEHp0P8LU8PrWWHEKUcKIQxEC
> CQv3mIVsiuS4yQIIDd7BlSarTFsCCQtwCEE1XYZ/KQIIBs84/O/aI4cCCQazNJyo
> Sa762g==
> -----END RSA PRIVATE KEY-----
>
> -----BEGIN RSA PUBLIC KEY----- MCgCIQC0uFXIkpfJTJNMCsc4UTHvkfUGgs1dW3r9OLEBSnNCVwIDAQAB
> -----END RSA PUBLIC KEY-----
>
> -----BEGIN RSA PRIVATE KEY----- MIGsAgEAAiEAtLhVyJKXyUyTTArHOFEx75H1BoLNXVt6/TixAUpzQlcCAwEAAQIh
> AJ9X/UNJhg4Wt7WPt1K/u/mPU9MwrXCqQVIPCXLrwuoRAhIA9DxAuEES2PbGPi5k
> 0hdwvk8CEAC9bNqhV9lSmjAn7XwlAXkCEgC7HsqrIFgzWGYsT9k4yXK8qwIPRsU1
> hZQRsKfEmBZFDHpZAhEv3CG7tGSFobuQzMFxO4643A==
> -----END RSA PRIVATE KEY-----

References
OpenSSL genrsa
Python RSA