Well, the movie deviated from the novel, in that when the cylinder opened the war machine was all ready to go. Since I'm sticking to 1953 film hardware, I want to keep the lifting body.
As for being fired out of a cannon, I have a scheme that may tie it all together: the lenticular entry vehicle will be encased in a cylindrical sabot or fairing, which will fall away as the entire stack exits the Martian atmosphere, revealing the entry vehicle and perhaps a propulsion module fo some sort for course corrections, which will be jettisoned shortly before arrival in Earth's atmosphere. My motive for this is that the sabot can serve as a first stage for a multistage.dll vehicle, and the propulsion module can be the upper stage. Under this scheme, the canon is just surface scenery, and the impulse delivered by the "first stage" will be delivered very quickly as it leaves the barrel. If this works, then we get a cannon, a course correction method, and a lifting body entry all in one.
ETA: once the basic setup is complete, anyone who wants to do a more traditional tripod-tech addon will be free to use the cannon and other related meshes. The more Martian war machines the merrier.