I think it's time a "Veteran" designer reveal one of the "secretes" how to create thickness for surfaces...
for a precise wall thickness and for lowering 3D Printing cost...
1. Google Meshlab and install it (its free)
2. load your surface model, better be an obj, stl ,wrl (DWG not supported)
3. click on the Filters tab--> Remeshing simplification.. --> Uniform mesh resampling
set the Precision parameter to 1.0%
set the offset value to 53.0%
check the "Absolute Distance"
seconds/minutes later you'll have some "meat" on your surface.. (exact X mm thickness)
if you check the "multisample" too , it will enhance the resolution of your new shell but could also crash the software..
when done, export to your 3d tool for cleaning and smoothing or.. learn how to do it on Meshlab too.