An implantable medical assembly includes a cover having a porous chamber. Enclosed within the chamber is a medical device through which aqueous body fluid flows from an outlet of the device into the chamber. The chamber is formed at least in part from a porous material that promotes vascularization and inhibits fibrotic encapsulation upon implantation and that allows aqueous body fluid in the chamber to flow through the porous material into body tissue in which the assembly is implanted. The chamber has a pair of walls that are spaced apart at a first portion of the chamber at or near the outlet and converge at a second portion of the chamber downstream of the outlet.