You've had loads of great advice but just thought I'd add my tuppence worth.
My boy is a real heavy wetter at night and I've tried everything on him, literally everything
I stayed away from fleece wraps for ages because I just didn't believe they would work but now I use them on him every night, my fav is the Dancing Bears wrap as it has a very high rise and is soft round the legs and they are generously sized so fit over any night nappy easily. They do an XL and I get mine from NappyMania.
I found that as I have to boost his nappies a lot many of the dedicated night nappies just didn't fit properly so I now use Stuffie nappies, the size 3 is huge and can take loads of stuffing without compromising fit round the legs. I either use a WeeNotions bamboo night insert and a hemp bigger weed insert or what was called the Babeco (changed their name a while ago, can't remember what to atm) night booster and the hemp bigger weed then stick the Dancing bears soaker on and a pair of fleece longies on top and never had a leak since. The nappy is sopping wet in the morning but no leaks.
Another option would be to buy a really good booster, Weenotions used to do a bamboo one with 3 layers of bamboo or even one of their night inserts and use it to boost the outside of the nappies you already use. This would be bulky and wouldn't fit under a standard wrap but the dancing bears soaker would work fine I would think. Then you wouldn't have to buy a load of new nappies.
Anyway hope you find a solution, sorry for going on a bit, I don't post for ages then, write a book