I use a bucket, and a 5 gallon paint strainer bag to put the strawberries in. I tie the bag and put it into the bottom of the bucket, then I rack the beer on top. They float a bit, but I don't worry about them floating or try to push them down. They eventually find their way into the beer anyway.
I used the 5 gallon better bottle once, maybe twice, before I wised up...first time I had a massive strawberry beer explosion that painted strawberries all over my pantry's ceiling and walls. Second time I had lots of trouble getting those little seeds out of the finished beer. So, nowadays I like using bags to contain and filter hops, spice and fruit additions, and I like doing that in buckets because they are much easier to manage with the bags.