Last week, the best my local supplier could get me for an in wall splice point was a plastic 3 gang deep box where I had to drill holes on the ends to put fittings in to splice 100amp SER with some Polaris lugs. I managed to make them fit, but with how stiff that wire was, I kid you not, I was using ratchet style c-clamps to push the wire into position in the box as I adjusted the positions if the Polaris lugs. Got it all to fit in with a lid with each cable coming in from the opposite end and screwing the box to a stud from the inside side wall with drywall screws. The lid fit, so it ships!

Not my favorite solution. What's your worst you've had to deal with?