Note: this proposed structure is very provisional aimed at stimulating comment. There is clearly some symmetry across the whole book. It seems that ch 28 is commonly regarded as the centre of the book and interrupts two speeches of Job each of which  begin with “Moreover Job continued his parable, and said …”. The speeches of Job’s three friends  balance the speeches of Elihu, but in the latter case there is no response from Job. Job’s opening lament may be balanced by the whole section from ch 27-31 but if so an explanation is still required for the structure of 27-31. The final two speeches from the LORD balance the two interrogations of the Adversary. 

A. Prologue (1:1-5)

B.i. First Discourse between YHWH and the Adversary (1:6-22)
B.ii. Second Discourse between YHWH and the Adversary (2:1-10)

C. Job’s three friends (2:11-13)

Job’s opening lament (ch 3)

First Cycle of Dialogue (ch 4-14)
Eliphaz (ch 4,5)- Job’s reply (ch 6,7)
Bildad (ch 8) – Job’s reply (ch 9, 10)
Zophar (ch 11) – Job’s reply (ch 12,13, 14)

Second Cycle of Dialogue (ch 15-21)
Eliphaz (ch 15) – Job’s reply (ch 16,17)
Bildad (ch 18) – Job’s reply (ch 19)
Zophar (ch 20, 21) – Job’s reply

Third Cycle of Dialogue (ch 22-27)
Eliphaz (ch 22) – Job’s reply (ch 23,24)
Bildad (ch 25) – Job’s reply (ch 26)

D. Job continues his speech (ch 27)
(27:1 Moreover Job continued his parable, and said …)

E. The excellency of Wisdom (ch 28)

D’. Job’s continues his speech (ch 29-31)
(29:1 Moreover Job continued his parable, and said …)

C’. Three friends replaced by Elihu (32:1-5)

first speech (ch 32:6- 33:33)
second speech (ch 34)
third speech (ch 35)
fourth speech (ch 36, 37)

B’. i. YHWH’s First Speech (38:1-40:2) – Job’s response (40:3-5)
B’.i . YHWH’s Second Speech (40:6-41:34) – Job’s response (42:1-6)

A’. Epilogue (42:7-17)