Welcome to the Bunnings Workshop community. Thank you for joining us and sharing your question about the suitability of perspex as a lid for a cold frame.
Perspex is the ideal cover for your cold frame on the condition that it is framed. Perspex is also known as Clear Acrylic Sheet. It's lightweight, tough, and durable. It's so strong that it can withstand hail storms that would normally break glass. It has superior thermal properties and provides the same light transparency rating as glass. Perspex without timber framing tends to bow in large sheets.
Would it be possible for you to post a photo of your cold frame? This will give our members a better idea of your setup and allow us to offer you a comprehensive recommendation.
If you need more advice or information, please let us know.