there is a Shapes tool in SAI2
Rasterize the layer > add in the tail > Ctrl+select shape > Shrink selection (the part where you wanna decide how thick the border is) > cut.
It sounds more convoluted than it is, but it's quite simple.
Also, i used to use some old Japanese plugin for SAI that let's you add different kinds of shapes but for the life of me, i cannot find it now : |
otherwise, you can always just use free apps like Krita, FireAlpaca, Mischief, MediBang...