As @ahaw021 said, you can use ZeroSSL in this case and it should work fine. (The "CRT" in this sense is the certificate and the "CA bundle" is the chain certificate.) However, this isn't necessarily very convenient because you'll have to repeat the process at least every three months—that's how long Let's Encrypt certificates last.
It would be more convenient for most people to get their hosting provider to implement some kind of more automated support for Let's Encrypt. Many providers have already done this.
For example, if your provider gives you a control panel interface to configure your shared hosting, the control panel might have a built-in feature or add-on to support Let's Encrypt automatically, which the hosting provider might just need to enable. You could also consider switching to a different hosting provider that has more convenient support for Let's Encrypt integration.