My first thought is an
electric drum module with a
foot pedal. However, those midi interfaces aren't as small as I suspect you want, and for something as simple (and somewhat trivial) as that setup, it'd be easier and a lot cheaper to buy a used drum machine and set it for a constant beat.
EDIT: Or, for a cheaper pedal, Rockband/Guitar hero drum kit pedals would work.
For an electronic unit, I'd recommend looking for a used Alesis DM-3 e-drum module. It's one rack space and has enough sounds and inputs to do a whole drumset. A hat just takes a simple switch like a cheapo keyboard pedal. The above expensive pedal is not necessary. if you want to "play" the hat you'll need a piezo trigger of some kind. These can be made from radio Shack piezo buzzers.
I think Alesis modules are up to DM-10 or something now ($$$), but I've got a pair of used DM-3s and they were about $100 bucks and work very nicely. The advantage of this rig is you have a very wide selection of sounds you can put on the pedal(s) Including hat, bass drum, snare drums, and a variety of latin things. The DM-3 is not programmable for sounds (I think that takes a DM-Pro) but the selection is VERY wide.
Personally the electronic thing has the advantage of a wide range of sounds, but it's some stuff to drag around. Personally I currently go acoustic. As others have noted you can get a standard bass drum pedal and mount a number of things to it. Mine can be set up for claves, cowbell, tamourine, and of course a piezo trigger to run the DM-3. Check out a drum shop to see how all this stuff is set up.
I bought a "porch board" stomp box from Rondo and that was a total waste of money. The big ones are, well, big and expensive too. And they only have the one sound not like the drum machine thing. Personally, I'd suggest using a real hat as being easier to drag around than the electronic rack! But still a real hat only does the one thing plus you may have to mic it.
Doing the simple high hat thing isn't bad but I can tell you that playing bass (or conga drums which I also play using the pedal) and doing left foot clave is a major trick! Still this is all so cool I really want to find time to try more with it. I much prefer to PLAY the percussion than have a drum machine play me.