Using a #75 or#80 in a pin vise,.. carefully drill many small holes thru the styrene,
where you want to open it,.. then even more carefully,
file or new #11 blade scrape, cut smooth out the remaining plastic until opened.
It helps to file, sand the underside to make it thinner ,..like paper thin to aid in the removal of the styrene,
Try it on a donor ,..first!
Cheers