Do not pour cement or mud, it wouldn't work (it would rob heat) and you need insulation.
You would have to make insulative ceramic (fire/refractory brick).

This detailed design guide
http://www.rocketstove.org/images/stories/design-principles-for-wood-burning-cook-stoves.pdf
explains how to make insulative brick.


You're basically making the version from this video http://rocketstoves.org/ so use dry, ashes or vermiculite/perlite from garden/hardware store, and cap off with lid.

Here is an oil barrel version, with insulative brick elbow
http://www.appropedia.org/CCAT_rocket_stove
and DRY vermiculite fill


Also see
http://www.aprovecho.org/web-content/publications/pub1.htm


IIRC, some reports (re)posted on www.bioenergylists.org say the tin-can version will rust after you fire it within a year (6months even), longer if you use black stovepipe. The ceramic elbows last longer.