Sometimes when you're working on old houses you find that the home supply stores don't have the molding profile you're looking for, or anything close. I have found that you can build custom molding by taking available profiles, or portions of profiles, of existing molding cutting them apart and reassembling them. Doing it this way you have a lot of variety in possible molding profiles.
Step 1: Select Your Materials
If you can't find a pre-made molding you like you can make all sorts of paint-grade molding profiles with various moldings available at your local home supply store. For this job I needed a plinth block. The plinth blocks available at the store we're not wide enough enough or tall enough for the old molding I was trying to come close to matching in a Victorian home. I selected some flat poplar stock and a piece of crown molding.
Step 2: Cut Out for New Profile
You can select different parts of the crown molding depending on what you want your profile to look like. Rip down the molding on the table saw to get your profile piece. Please note that the image has my door casing just to the left of the molding I'm using to build my plinth. This is the casing that will sit on the top of the plinth when finished.
Step 3: Pieces of the New Plinth
I used two pieces of the popular stock to build up my thickness and stacked my cut out crown molding profile on top. Here's what it looks like.
Step 4: Installing
Next you can just glue all the pieces together as you install it using your brad nailer and white glue.
Step 5: Fill, Caulk and Paint