I see a lot of plans, but they were most times made of knex and hard to build without electrical tools, thing I haven't.
I choose a sandwich format with a very simple trigger.
to build this I used some planks, screws, nails, a spring, a piece of cord and a construction Iron 6mm, I think.
DON'T use a 6mm construction iron becouse it's too thin. use one about 10mm (1/3 inch)
Step 1: Pieces.
anyway, it's about 50cm long, 6cm wide (not counting with the iron) the iron is about 50cm long.