Using the pline option is the simplest but the hard part is if you want some form of shape around your block. There is software out there that will draw a pline around an object, a common description is a boundary of objects but some methods are better than others. I would start with googling "boundary lisp". There is one version of software out there that appears to be good but you have to purchase.
A rectang around a block is simple use boundingbox.
