Depends on body type.
You MIGHT try a cable tie round each wrist tight enough to stop it slipping off then another cable tie between the two.
If you are slim and middle cable tie is not pulled too tight, you could fasten your wrists in front and slip them under your bum.
If you're dexterous, you could try joining the first two ties behind your back.
Personal opinion: don't use cable ties, they hurt!