It’s probably been done several times before but I can’t find it…..

Which 10 speed rear mech works best with a 42t expander and a long B screw?

Not exactly what you are asking, but the SRAM X1 that came on my Capra works fine with my 11sp 9-44t cassette

I use XT but use the one up cage plate to offset the top jockey wheel. This keeps the B screw adjustment in check.

Works like a proper one

I have used both SRAM (X9 Type 2) and Shimano SLX with One Up RADR cage and both work pretty well.

I prefer the clutch on the Shimano and think with the RADR cage works better than the X9. The X9 is no longer available and has been replaced by GX, but can’t comment on that directly…

depends what you’re running at the front…

Just get a One-Up components RadR cage and you don’t need the longer B-Screw, any Shimano Shadow + mech will work then, just pick your budget.

I ran a Deore M615 with the RadR cage and it was flawless

Probably a 32t.

Didn’t really want to change the cage – it’s for the lad’s bike so trying to keep it fairly cheap.

Could run him standard 2×10 but a 42t as a bail out and not having to mess with a front mech is appealing.

Already have a spare cassette / expander so just need a mech + shifter

Probably best to look for a cheap Sram NX/GX or if you are lucky X1 mech and shifter.

shimano 10 speed will do it but will always be compromised. Or if you don’t mind a fettle and have a shimano mech already then drill and tap a new M5 hole just behind the pivot point on the outer cage and move the jockey wheel backwards. Its tight but I think it could be done and won’t ruin your mech trying, never done it so not sure if the outer cage will foul on the cassette in biggest sprocket but shouldnt as the cassette drops away and we are only talking a few mm. Might need a small shim making to set the cage width too but that is easy with a washer and file to take the edge off.