Placing a magnetic shield in front of Mars to protect its atmosphere from the solar wind seems far more practical.
You take the entire planet apart and use the entire mass. It contains large amounts of nitrogen, carbon monoxide, water and stony material. Use the energy of D+D fusion to do the job. Fabricate a small megastructure (compared to Niven's Ringworld or Dyson's sphere.) that never the less have much more floor space than the inner solar system.
I imagine it shopping malls for millions of miles as is shown in some O'Neill graphics and also Wall-E. Some people would find it comfortable.
It would be a big space and have some room for "nature". It looks hard for me to see how to provide relief like a mountain range unless the relief is axisymmetric.
I sketched out a very small Bernal sphere for a showy space hotel where it would be quite easy to climb up to the low-gravity "hub". It needs 15 starship loads of N2 for the atmosphere - pluto has a lot of N2 and could enable those bit O'Neill airspaces.