Corners are not truncated much. In order to get 90 phase shift, you need to truncate more to generate 2 separate X-Y resonant modes such that when fed between each axis you can get more circular quality with desirable axial ratio of 3dB. Consider truncate corners by 25% and feed off-center.
more general info but not 1/4wave **broken link removed**
**broken link removed** example of circular path, bottom feed.
Better design two orthogonal antenna with diversity switch to choose best.