I just bumped into this free software and thought some of you might like to give it a try. I have not tried it at this time. It seems available for different operating systems. Some O/S might need a compile step, but for the price, it could be worth the trouble.
There may be alternative ways of creating custom interlaces with InkScape and other such tools. But, once we can get Knotter going, this might be a real easy way of doing just about any interlaces / Celtic knot (?)
As the author of the software says at http://knotter.mattbas.org/Knotter :
Interlace patterns are a kind of design historically used as a decorations in many places and by different cultures. Some examples are Celtic knot works and Islamic interlaces.
Knotter aims to allow its user to intuitively design such patterns and to provide easy ways to integrate the result in external general-purpose graphic software for this purpose designs created within Knotter can be saved in a custom Human-Readable format and exported as scalable vector graphics and in a wide number of raster image formats.
The software can create knots based on an image :
Or various parameters to make your very own Celtic knots or interlaces :
Seems like one of those oldies but goodies Creative Commons piece of software which could come in handy for the sign maker hiding in each us.
Have fun ...
Knotter : Make Celtic knots and custom interlaces 2015-12-22
Knotter is a free and open source customizable interlace designer made by Mattia Basaglia