1. Let Labour pick who is PM, *but can't be Corbyn*
2. Let Tories pick who is Chancellor, *but can't be Johnson*
3. Labour fill 3rd most important role (could be Corbyn)
4. Tories fill 4th most important role (could be Johnson)
6. LibDems fill 6th
7. Labour, Tories, SNP, LibDems next (10 Cabinet posts filled)
8. Change UK TIG, Greens, Plaid Cymru next (13 Cabinet posts filled)
9. If need be, another couple of Tory/Labour rounds
10. Hey presto, a real Government of National UNITY.
PM: Labour
Chancellor: Tories
4 more Labour Cabinet Ministers
4 more Tory Cabinet Ministers
2 SNP Cabinet Ministers
2 LibDem Cabinet Ministers
1 Green Cabinet Minister
1 PC Cabinet Minister
1 Change UK TIG Cabinet Minister
That's 17 roles. More than enough!
NOTE 2: For "Tories" you might also include "rebel Tories".