There has to be large quantities of heroin in evidence lockers.Why not just use that?
Strap them down. Inject a mild sedative. Strap on an oxygen mask, introduce some nitrous oxcide, once asleep, switch to inert gas. Asphyxiation takes a minute or so. Keep the inert gas flowing for 45 minutes or so to ensure.
Inert gas such as nitrogen is easily obtainable, inexpensive, and it works.