Hi,
Greetings and Welcome to Intel's Forum.
After creating your custom IP, Intel recommends that you move _hw.tcl files and their associated files to an ip/ directory within your Intel® Quartus® Prime project directory.
Reference: https://www.intel.com/content/www/us/en/docs/programmable/683364/18-1/save-an-ip-component-and-create-the.html
If you happen to move the files, point Platform Designer to their location with Tools menu -> Options in Platform Designer GUI.
You may have a look at this documentation for more information on creating your own custom IP: https://people.ece.cornell.edu/land/courses/ece5760/DE1_SOC/Making_Qsys_Components_15_0.pdf
Let me know if this helps.
Thank you.
Regards,
Kelly Goh