How many copies do you expect to sell? I doubt if any password protection is worth the trouble for small obscure scripts such as Model Builder. You can describe the terms in the licence agreement and rely on that to get commercial users to pay.
Since you won't get rich by selling scripts, why not just contribute it to the user community as open source and charge for support? Commercial users are likely to be much more interested in your expertise to customise the script for their use than hack it to understand how it works. Consider it advertising for your skills.
I have never attempted to hide my scripts, in fact I always provide all the source code for all my work and am still busy. There is always something new to develop even though I still use some scripts that are 20 years old, and I assume my customers may be doing the same. They probably wouldn't be able to do that if they were password protected because no one would remember it.