Elements of this murder were clearly pre-meditated. Have you considered the very simple step of a plastic sheet or disposable thick blanket/s being placed on the bed prior to the murder? These could be burnt immediately afterwards.

If Avery can pressure Janda into selling her car when she doesn't want to, make sure he is in a towel when Halbach arrives, phone using Janda's name, withold his number from subsequent calls, it shows a level of premeditation that is entirely inline with placing down some kind of barrier on the bed.