or make a wood jig to clamp the acrylic to while it cools. Thats what I've done on the couple of overflows and odd shaped frag racks I've made and they've come out great.
I was about to suggest that. IME the acrylic still tends to relax slightly after release from my jigs, but it's not much.
I think the mild curve, and then rounded corners on the biocube will be the thing that makes it hard to match the curves just right.