Simple ramp that I designed for my Xiaomi cleaning robot to ride over the door threshold. You can stack them with two M2X10 (or longer) screws or glue them together. I personally glued them to the threshold with double-sided tape, but there are also slots for screws so you can mount it to the floor.

You can simply resize it in your slicer to fit your threshold height. 

IMPORTANT: Some robots will detect black filament as a gap and will not pass through the ramp because the cliff or fall sensor does not receive sufficient IR reflection. To avoid this, please consider printing it in a colour other than black.