Bass amp = bass boom.
If you dont have one, turn the bass up an the treble down on your guitar amp.
I use a piezo in mine, encased in a sensible chunk of hardwood. I then silicone this into my box, and fill all the remaining space with foam cut from a camping mat, all siliconed into place. I usually place the piezo block over to one side, so one edge is in contact with the box side - hitting it here gives a good 'bash' sound, hitting it elsewhere, where the piezo block is protected by the foam, gives a duller thud.
Try experimenting with different coverings on the top of the box too..... foam, carpet, tin, plastic, metal... anything youve got!
Hope this helps! :o)
I make stomp boxes with different types of pickups and they all get the job done, even through a MicroCube. My Terraplane has a license plate as the top and an actual bass guitar pickup inside. The Angel Stomper has a Tesla Transducer piezo pickup and really rocks. I've used bare microphone elements and some folks even use a speaker as the pickup.
Here's a tip, if you are getting feedback, place a sheet of crumbled up newspaper inside the box.