fabrics that nappy covers are made in:
- PUL
- Malden mills fleece
- wool soakers

closures
- aplix (an industrial strength velcro)
- snaps either front snaps, or side snaps. I found side snap covers more adjustable, on my child.
- wool soakers have a drawstring or elasticated around waist
- there are MM fleece covers (like bear bottoms) that don't use any closures at all. See the Monkey High pattern on Fairy Fabrics.

If you are a sewer, sewing with MCN fabrics is different than dressmaking fabrics.
To make covers, you will need swimwear elastic, a walking foot, i'm rusty it's been years since i thought about this stuff, but i remember, there were "tricks" to it. Sites like NappiesCovered have links for patterns (and useful tips). Nappycino had lots of great hints too for DIY'ers.

Whether you buy or make the covers, i think to put covers over a terry flat, you would need a cover style with a long rise.

All the best as you enter your third trimester, i wish you a safe and healthy pregnancy.