Here are two Paint Shop Pro scripts that will let you turn any stripe pattern into a seamless plaid tile.
Instructions are included in the zip file.
These scripts were made in PSP 8 and should work in all later versions. They don't work in Photoshop.
They're not hard to use but you should at least know your way around the program interface and understand the scripting tool.
-Attribution is requested for personal, non-commercial use.
-Attribution is required for commercial use. (Please use http://psp-works.deviantart.com or link directly to this post.)
Full TOU can be viewed here and is included in the zip file.