The term ad hoc can convey a lack of rigor, for example one of the synonyms is to improvise.
Are there synonyms that do not convey this lack of rigor? For example, 'ad hoc' is used to refer to reviewers of scientific manuscripts; the Journal of Immunology states
AD HOC REVIEWERS: The Journal of Immunology would like to thank the many scientists who reviewed manuscripts in a conscientious and objective manner. The continued high quality of The JI depends upon the dedicated service of all of these individuals.`
Here is an example of a use that I would like to replace 'ad hoc' with a word that does not have casual connotations:
When I want a particular type of widget, I go to the store and get one. If the particular widget that I need is unavailable, I create one ad hoc.