The Lego advent calendars come with these inserts that are perfect for starting seedlings.
The final newspaper pot will be the size of a toilet paper roll, so this is how I checked to ensure the plastic tray would have big enough compartments.
I marked on the toilet paper roll the depth of the try compartments.
Then I cut three or four inch wide strips of paper.
I rolled a single sheet strip, but when it rolled up, the pot was multiple layers thick.
Then I just folded over the bottom.
No Guff Gardening site.
~Miss Greenish Thumb~