Many amps break fine with no problems. The ones prone to fracture, cut fingers, and wasted gear are inadequately scored. Sticking the fracturing non-scored amps in a biro lid or other hole as mentioned above will not save your fingers or the gear if the amp fractures into shards.
Best bet is to sellotape the entire amp up to the neck and score slightly with a glass cutter. Then stick it in a hole or biro lid.