Poo leak down mid back with Huggies Ultra Dry

Hi

Our kid is 3.5 mo old , ~6.5kg, My wife uses disposables and we use Size 3 (6-11kg) Huggies Ultra Dry (there doesn't seem to be any difference between ultra dry and crawler) When he was younger we were using Huggies Ultimate and didn't have poo leaking out the back. I bought about 3-400 Huggies Ultra Dry on special last month as I thought they were the same as ultimate.

Now he is older he has one or two big poos daily (passes stool on average 3-5x daily) and he has poo leaking out the back almost once daily. The amount of stool when it leaks is usually enough to go through both his inner onesies and his romper. He has very watery stool as he is breast fed.

No problems with urine leakage.

When you look at the nappies there is a gap that is not tight in the small of his back at the rear.

I was wondering about buying a re-usable outer cover to bolster and tighten his disposable diaper at the mid of his back at rear to prevent leak and also contain leak if it does occur.

What are people's thoughts? My Wife (who does the bulk of the work) would prefer to stick with disposables doesn't want to go into reusables.

Thanks