Shiva Purana
मंडलं चतुरस्रं वा त्रिकोणमथवा तथा । खट्वांगवन्मध्यसूक्ष्मं लिंगपीठं महाफलं ६ ।
The seat can be circular, square or triangular in shape. The one shaped like a cot in the middle is of middle efficacy.
english translation
maMDalaM caturasraM vA trikoNamathavA tathA । khaTvAMgavanmadhyasUkSmaM liMgapIThaM mahAphalaM 6 ।
hk transliteration by Sanscript