try density in the rect function - an example
plot(c(100, 200), c(300, 450), type= "n", xlab = "", ylab = "")
rect(110, 300, 175, 350, density = 5, border = "red")
you can use lty to have different types of borders and shading
example -
rect(110, 300, 175, 350, density = 5, border = "red", lty =2)