we used to get the yellow plastic caps that you could get in rings for cap guns that repeated, you could scrape/squeeze the stuff out of them, then put it all in a pen top and that would go off well.

I remember doing one at home and my old man collared me, thought I was in the shit but when I told him what it was he laughed and said it wouldnt work and to prove it he would take it out in the garden and show me, he leant over it and lit it and it went off in his face