Frankly , there is nothing to it in making a drum trigger.
All you need a piezo electric sensors that are maybe 20 pence each or whatever the equivalent is in the US. You could probably find them in SIM LIM. Try Bytetron.
All you do is stick in on a surface and wire it to a drum trigger module or a midi to trigger module.
If you make a cymbal 'trigger' you can use plastic practice cymbals from Pearl and mount some gum rubber on the top. If you are making a drum 'trigger' you could mount it in or over the drum.