A quick online search reveals:
147 mg/kg (LD50, Rat, oral, Doxepin)
16 mg/kg (LD50, Rat, i.v., Doxepin)
135 mg/kg (LD50, Mouse, oral, Doxepin)
26 mg/kg (LD50, Mouse, i.v., Doxepin)
147 mg/kg (LD50, Rat, oral, Hydrochlorid)
13 mg/kg (LD50, Rat, i.v., Hydrochlorid)
180 mg/kg (LD50, Mouse, oral, Hydrochlorid)
15 mg/kg (LD50, Mouse, i.v., Hydrochlorid)
LD50 means 50% of experimental animals died at that dose. Aside from that, Doxepin is a TCA which are known to be lethal after an OD.
Klonopin won't do the job afaik.
This not a medical advice. Please also do your own research.