I've tried all sorts of DIY fixes to rectify the issue, including at one point grabbing a log of wood and stuffing it between my pedal set and the wall. Just the right sized log would snugly fit for a decent setup, however, wasn't immune to Dark And Darker Gold the odd slippage now and then. Also that only really worked in the setup I had at the time, and now I'm nowhere near a wall to prop the pedals up against.
For all the high-end sim racing cockpits out there, including the Playseat Trophy Logitech G Edition I'm also testing, it turns out a foldable racing wheel stand such as this one from Monoprice was all I actually needed. It's in no way as snazzy or comfortable over long periods as the Playseat, which I'm sitting in as I type this, and it won't place you into a proper racing position, either. But it's pretty affordable, fits my wheel comfortably, and conveniently folds down to a storable size.
The most important thing is that the Dark Matter GT stays in place when I slam on the brake pedal. I've screwed the T3PM pedal set from the Thrustmaster T248 to the base and now it's firmly locked in no matter how much I stab at it.
Honestly, it's brought back a lot of joy in sim racing for me, and I didn't entirely grasp how much pedals that move about all over the place under braking affected my desire to get my racing wheel out until now.
As for the Dark Matter GT, it's a fairly stripped back concept. There's no chair component, like you would see on many cockpits, so you'll have to Darker Gold provide your own (may I recommend one without wheels). It's simply a foldable frame for the wheel base, pedals, and a shifter, if you have one.