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.pdfexplains 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_stoveand DRY vermiculite fill
Also see
http://www.aprovecho.org/web-content/publications/pub1.htmIIRC, 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.