There are different methods for supporting the ribs whilst making a bass.
Some luthiers use a bottom board alone whereas others use some form of mould.
The one I use is based on the design shown by HS Wake and consists of three sections separated by spacers. The top and bottom sections are identical and follow the profile of the body of the bass with sections cut out to receive the corner blocks and top and bottom blocks. The middle section is shaped to take steel angle brackets used to support the blocks whilst the ribs are being fitted. The blocks are made from European Spruce and must be profiled to fit the outline of the ribs. Making an accurate convex profile like this is not easy and the wood can prove difficult to hold.