Hi Nikki,
Yup, square knots slip.
Try this adaptation of a surgeon's knot:
First, do the basic surgeon's knot - left over right and under twice. At this point, the typical surgeon's knot ends by pulling the knot tight but this doesn't work when you're tying the knot in a drive band...
so, to make it work, the next step is to go right over left and under once. Now it will pull tight and stay put.
Or, in other words, it's the basic square knot with an extra "left over right and under" on the first step.
Llyn