Non paid solo-coding assessments should disappear. Many disrespectful companies send it automatically to all applicants to don't even look at them after.
Every time I get those I request to do a live coding assessment, if they are not willing to lose even 1h of their time they were not serious about you.
If you decide your process really requires a solo assignment you can offer a gift card to prove you appreciate the candidate (and don't be stingy, you are already saving a lot of money by not having 1 of your engineers interview)