Draw in PowerPoint, then
"Save As"a Portable Network Graphics (.png) file, with resolution at 300 dots per inch (set via "Options" button in "Save As" dialog).
Open the page (e.g. Slide1.png) with GIMP. Go to
Rect Selectto select the rectangle you want. Then select
CopySave the image subset by clicking
Newin GIMP (makes a file the right size), then choose
Save As...in GIMP, and choose Encapsulated Postscript (.eps). This saves the rectangular region as a .eps file of high quality.