You've got the theory of operation backwards. Bells pose very little drag whereas a pipe system has the potential to put much more, especially when many bends are included in the run.
A good comparison can be read here: wood-heat-storage-flues-vs-bells
Floor of the bell should not matter how bumpy it is as it won't put drag on the system. Its called the Free Gas Movement because the exhaust is like water and seeks its own level, stratifying into all these nooks and crannies, not scraping by them.
Again, cover your exposed half barrel bench, wait for a cold enough morning and let her rip!