My son is a heavy wetter too & sandy's worked good with a meaf cover but the meaf is bulky & fit sorta weird. Meos were too thin.
If you have fleece pjs, I would suggest lollidoos! They are a fleece pocket fitted. We stuff it with 2 flips prefolds when he was a super heavy wetter & even though it was very bulky - it worked!
Also it didn't require a cover. Which scared me for sure but truly doesn't need a cover esp with fleece pants or pjs over.
Only downside is compression leaks with elastic pants.