Floating is not desirable. It really should to be touching the bottom with a preferred slight snug fit to do it's job with full effect. I have a few that are lose and a few that are tight but none of them float. That being said you have a few solutions.
Home remedy:
Take 4 zip ties and attach them to the outer ring. Place them right next to the supports that are touching the bottom of the bucket, zip them tight and cut off the tail. This will increase the circumference of the grit guard for a tighter fit. You can also adjust the fit by using different size zip ties.
Another solution $:
Hit up Home Depot for a 5 gallon Homer bucket. Grit Guard fits nice and tight. While you are there I would also recommend getting a Gamma Seal Lid.
$2.60 for bucket
$6.97 for Gamma Seal Lid
Cheap, easy, one stop with no shipping charges and no waiting.