I recently started working for the county (I used to work construction) and we send out large amounts of notification letters (I also plan on using this when I send out wedding invitations in the near future!!). I asked our secretary if we had some sort of letter sealer and they did not. Staples wants $17 for one similar to this. I looked on thingiverse for something similar but I found none. I designed this in Autocad to the size I thought would work. After a first messed up print I decided I could cut the size in half. PLS NOTE YOU WILL DEFINITELY NEED TO SCALE THIS OBJECT DOWN.
I printed this at a finer resolution and than acetone vapor bathed the base part to make sure no water leaked out. Do whatever you feel you need to.