Foxborough should be able to cover three-quarters of the cost of a new Town Hall with existing funds, officials said at a meeting last week, leaving about $2 million of the projected $8 million price tag to be covered by a temporary tax increase. That would average out to a hike of about $390 a year over 20 years for the average home assessed at $376,000, finance director Randy Scollins said during a presentation. Special Town Meeting voters on Nov. 4 will be asked to fund the $550,000 needed to design the new municipal facility and would go to the polls again in the spring to approve the actual project. One way to reduce the tax increase from construction could be to apply some $600,000 in available meals tax receipts to the project, board chairman Mark Sullivan said. Those funds are usually designated for insurance issues and roads. “That shows we are throwing all our eggs at this project,’’ he said. Selectmen took no action on the issue.