Does A Sirtuk Need Tzitzis?
If a Sirtuk with four corners has one of its corners folded and sewed so that it appears rounded, is it acceptable?
Also, can I try-on a non-rounded Sirtuk in the store?
As you correctly indicated, a Sirtuk—as is the case with any four-cornered garment—cannot be worn (unless Tzitzits are added).
Folding the corner as you mentioned, however, is not acceptable since the Sirtuk still retains all its corners (only that one is folded). The common practice is, therefore, to cut off one of the corners in such a way that it is rounded enough to be exempt from Tzitzis.
[See here for the minimum rounding required.]
In any case, you may try-on the Sirtuk [see #1870 that one needs to derive “benefit” from clothing in order to require Tzitzis even in the case where one owns the garment and certainly when trying one in the store].
אדה״ז י, ו.