You can really only CTB on benzos alone if you already have an existing condition that affects your breathing such as sleep apnea, then it might be possible to get respiratory depression when your breathing problem happens when severely overdosed on benzos.
One hundred and ten cases of suicidal suffocation using a plastic bag were identified in the files of the Office of the Chief Coroner of Ontario, Canada, between 1993 and 1997. The records were reviewed to determine the demographic characteristics of this group compared with all cases of suicide...
pubmed.ncbi.nlm.nih.gov
According to the link above, apparently it is possible to suffocate with a plastic bag while under heavy sedation from benzos. I personally still don't think benzos would be strong enough, and the suffocation is more likely to wake you up while you panic and rip the bag off your head.
I've also heard of people overdosing on benzos and alcohol, then drowning in their bathtub. I still think that's unreliable for 2 reasons:
1) While alcohol does potentiate benzos, it also increases your risk of vomiting without an antiemetic
2) Bathtubs aren't tall enough for my tastes. A lake that's deeper than your standing height would be more reliable. Maybe with an anchor too just to make sure your body doesn't float to the top.
Along with opioids, if you can also get your hands on a barbiturate (Phenobarbital will do the job just fine), that'll help greatly too.