A bell rather than ducted bench mass is probably a good idea, with limited mass. Making the bell long and wide will spread the load over the most supporting beams and walls below; keep to edges and well-supported areas as Gerry says. You do need to raise the mass off the floor a few inches to keep heat away from wood and allow cooling air to flow underneath it.
The internal surface area of a bell in relation to the combustion core size is critical; you can find information at
batchrocket.eu, under "Sizing a batchrocket" and "Bell sizing".