What should I do if I lost my 90-day report slip in Thailand?

I never was asked to show my previous 90 days confirmation when doing my report in person. They have all the details in their system.

You will get an email reminder approximately 1 month before your next report is due. This way you'll know the date, unless you went abroad in the last 2 months.

That is gonna go one of two ways

You'll either be able to do the next one in person without having the previous 90 day report receipt

OR

They're going to tell you that you need to go get a police report stating you lost it, THEN they'll let you do the next one

Hopefully you have an idea when your next report is due

I would say if you've already done one in person, try to do the next one online using the web-portal (which you can do 2 weeks before the due date down until 1 week before the due date) because that way doesn't matter if you have the old receipt or not..