I just write them out by hand most of the time. Or if I have a class very similar from another project I'll copy/paste it.
Another option is to create a separate project with the full Laravel project. Use Artisan to make your controllers and models, then just move the files it generates into your Slim project. You wouldn't pollute your Slim project with any additional projects, you would have access to the Artisan generators, and the only extra time would be the second or two to copy the generated files from the fake Laravel project into your Slim project. You could even keep that Laravel project under version control to easily wipe our all of the generated classes after you have copied them to the Slim project. I might actually do that myself.