Have you checked out the Razer Wolverine? I don't know that it can be mapped to keyboard keys but a workaround like the steam controller configurator might work to override the ABXY button outputs.
You could also try using a foot pedal, in conjunction with the controller. I recently picked up this one after my old school Fragpedal recently died. Seems to work great and I can map it to any buttons I want.
I am no foot pedal expert, but what makes this the first Gaming Foot Pedla haven't there been others because I have one that accommodates 3 pedals on one and I'm looking to get another because they work just fine.
There are a bunch of other models online too. Do they just mean the of that style with the left and right flexing which in my opinion is the most interesting aspect, or the LEDs, or made by the MSI brand?