any switch pedal will do…
ideally, you’ll buy one with two pedals, that has a stereo output - as this is supported by hapax, as two different “actions”
I guess, you could also buy two single pedals, and then use a 2 x TS-> TRS adapter…
not done this, but perhaps this is useful if you have two master keyboards… and so want to control from two different pedals.
note: pedals have polarity (i.e. open or close on down), but I think all pedals allow this to be configured.
I use a Boss FS 6 - which I like a lot 
(you can look at it for specs)
I’ve not tested with Hapax, as Ive got it hooked up elsewhere… and currently the Hapax is sitting on my main keyboard, so I don’t really need a pedal.
but Im sure it works with Hapax (I’ll test if you need), as its worked flawlessly with the Pyramid, and synths.
also, as stated above, we currently have only a few actions we can assign to the pedal (unlike pyramid)… though in your case, all you need is record, so it’ll be fine!
why FS6?
dual switch, which can be momentary or on/off.
has stereo output which can directly go into Hapax/Pyramid
(also has mono, which is useful for other synths etc)
battery powered (*)
its a quality product… had it for years!
(*) theres a pro/con here…
the FS6 is quite ‘clever’… when it does not have a jack plugged in, it powers off… so you can leave a battery in it, and it wont drain… so it doesn’t have/need a power on/off switch.
but, sometimes I forget to unplug it… and so drain the battery.
that said… in fairness, I guess if it had a switch, Id be just as likely to forget to switch it off 
I think the other thing perhaps Squarp could consider is supporting MIDI pedals for these kind of operations.
whilst generally, something like the FS6 is idea for this… we are of course limited to 2 foot switches, which for looping purposes can be a bit limiting.
e.g. I’ve an old Keith McMillen Softstep… with this you could assign many more functions - play, stop, track select, undo , snapshot etc.
the other option I guess, is for Squarp to get more sophisticated with the pedal input…
so alot of hardware loopers, also only one or two switches.
however, they have quite a lot of gestures… short press/long press/double (or even triple) tap, which are also often ‘context sensitive’ (so whats is currently happening)
though in fairness, this may be expecting a bit much (at least in early days) …
this pedals are 100% dedicated to looping functionality, so its not surprising there excel in this area!