I welded one up out of a piece of scrap metal and used a bearing press bushing sleeve of the right size to press it in...worked great.
However, someone shared a 3D printed version. I will try printing one tonight. Happy to send one along if it looks decent to see if you can use it. If you have the exact outer diameter of the seal I will double check this design. PM me.
https://www.printables.com/model/205318-porsche-944-968-rear-main-seal-installation-tool
Mine:
