I believe that you heat up the plastic with hot water and then chop it off with garden shears or simply bend it off. Make sure to soften any sharp corners. You also may not have to do it. I have two boars, one full grown and one five month old, and both of them can fit perfectly in a small pigloo.
Our pigs love stuffing themselves into the pigloos. I have two full grown boars and they are sometimes happy to share the same one. You'd be amazed the spaces they can squeeze into. (Especially behind the couch, be careful!!)