DIY and hobbyists’ stores that sell materials for amateur dolls-house-makers often have very nice ready-made staircases for those who find it too fiddly to make their own. You’ll need to buy three of them. Then paint one of them gold and one silver, decorate the third with imitation gems and glue them all together.
Voilà! You now have your own aniconic Sankassa Staircase image, just like the one on the Bharhut stūpa.
From the entry for “Sankassa” in the Dictionary of Pali Proper Names:
A city, thirty leagues from Sāvatthi. (DhA.iii.224). It was there that the Buddha returned to earth, after teaching the Abhidhamma Piṭaka in Tāvatiṃsa, following the performance of the Twin Miracle (Yamaka Pāṭihāriya) under the Gandamba tree. As the time approached for the Buddha to leave Tāvatiṃsa , Mahā-Moggallāna (Anuruddha, according to SNA.ii.570; cf. Vism., p.391) announced his coming return to the multitude, who had been waiting at Sāvatthi, fed by Cūḷa-Anāthapiṇḍika, while Moggallāna expounded the Dhamma. They then made their way to Saṅkassa. The descent of the Buddha took place on the day of the Mahāpavāraṇa festival. Sakka provides three ladders for the Buddha’s descent from Sineru to the earth: on the right was a ladder of gold for the gods; on the left a silver ladder for Mahā Brahmā and his retinue; and in the middle a ladder of jewels for the Buddha. The assembled people covered the earth for thirty leagues round. There was a clear view of the nine Brahma worlds above and of Avīci below. The Buddha was accompanied by Pañcasikha, Mātali, Mahā Brahmā, and Suyāma. Sāriputta was the first to welcome him (followed by Uppalavaṇṇā, SNA.ii.570), and the Buddha taught the Dhamma, starting with what was within the comprehension even of a worldling (puthujjana), and ending with what only a Buddha could understand.
On this occasion was taught the Parosahassa Jātaka (q.v.) to proclaim to the multitude the unparalleled wisdom of Sāriputta (DhA.iii.224 ﬀ; see also SNA.ii.570). It is said’ that the Buddha’s descent to Saṅkassa had provided opportunity for Moggallāna to show his eminence in psychic powers, Anuruddha in the divine-eye, and Puṇṇa his in skill in teaching, and the Buddha wished to give Sāriputta a chance of shining in his wisdom. (Ibid., loc. cit. J.iv.266; see also Jhānasodhana, Sarabhamiga, and Candābha Jātaka). He therefore asked of Sāriputta questions that no one else could answer. The opening words of the Sāriputta Sutta (q.v.) are supposed to refer to this descent from Tusita (sic). The site of the city gate of Saṅkassa is one of the “unchangeable” spots of the world (avijahitatthānaṃ). All Buddhas descend at that spot to the world of men after teaching the Abhidhamma (BuA.106, 247; MA.i.371, etc.). From Saṅkassa the Buddha went to Jetavana (J.i.193). A shrine was erected on the spot where the Buddha’s right foot first touched the ground at Saṅkassa (DhA.iii.227). When the Chinese pilgrims, Hiouen Thsang and Fa Hsien, visited the place, they found three ladders, which had been built of brick and stone by the ancients, to commemorate the Buddha’s descent, but the ladders were nearly sunk in the earth. (Beal, op.cit., i.203; Fa Hsien, p.24).