Hi. In 1977 I polymerised methyl methacrylate. Easy for me to say. Did it in a brand new boiling tube. Perfect cast but we were made to cool them for about 4 hrs.
To reducing warping, you can add a moisture-resisting material which has an acrylic substrate and from a group of PVDF homopolymers and copolymers etc, to form a low moisture absorbing multi-layer object, and thus preventing warping from happening.
All these acrylic sheets have an disadvantage, which is it tends to absorb water in humid conditions, thus causing the exposed side to expand significantly, causing bowing to happen. To prevent that from destroying the object, acrylic capstocks was used to improve the weatherability of the acrylic sheets.