We already have them. Inmates in most US jails and prisons are charged a per diem just for being there. It’s disgraceful.
If someone is unable to pay their debt, they are thrown back in jail, accruing more debt. How is this not a debtors prison? Perfect examples are listed on this wikipedia article. Child-support, garnishments, confiscations, etc. It's exactly what I described.
https://lasentinel.net/if-youre-poor-in-america-debtors-pris...
Inmates who are unable to pay their fees (for even being in jail) are unable to leave. For-profit prisons are real.