What should I seal it round perspex with?
Why would you sand or seal it?
A bit of thin, galv sheet steel, or alloy, cut to size would be my choice for longevity. Four 6mm roofing bolts and nuts, would be adequate, to fix it in place. Drill the sheet corners first, then drill the bin to match.
Why is this thread still active, and the deed not completed, six weeks after the OP?