Running wire to sub panel in shop 80 feet away
Make the splice in the panelboard itself

I'd use a two-pole aluminum rated power distribution block of suitable size listed to UL 1953 and rated to IP20 (such as a pair of Eaton/Cooper PDBFS303s, dovetailed together) to make the splice inside the panelboard cabinet instead of in a separate junction box.  Then you can run a pair of 2/0 jumpers from the distribution blocks to the panelboard terminals.

This takes up a bit more panelboard cabinet space, but is neater and cleaner than having splice lugs hanging loose in the panelboard or having a separate box just to splice wires.  Check with your AHJ first though, as there is the off chance that they're not OK with mounting things to a panelboard cabinet that the manufacturer did not originally intend to go there.

(Making the splice in a separate box is also possible, just slightly silly as you'd need more wire and parts to do that vs. doing the splicing in the panelboard cabinet.)