inside dimension for homemade digestive cookie mold
The dough used in a mold has a lot to do with the end result. It's best to use a firm, non-sticky dough, that can be gently pressed into the mold to imprint the design.  Commercial digestives are not baked in a mold, but are machine embossed and cut before baking, using a rotary mold that incises the distinctive design (e.g., Digestives, Oreo). For your biscuits, you might consider creating a stamp or, if you're really ambitious, a rolling pin version.

Here's an example of a stamp sold at Jarrold (UK)


And a commercial roller manufactured by F&amp;S Engraving (Mount Prospect, Illinois US)