LED panel wire connection space
Here's the normal way to do that, which your LED vendor was expecting you to do.
Bring your Romex cables into a 4 x 4 x 1-1/2&quot; metal box ($1) attached to the actual ceiling.  Sheath must extend at least 1/4&quot; beyond the cable clamp.  Wires must extend at least 6&quot; beyond end of sheath.   Box must be within 5' of the light's attach box.
Install a 10-32 ground screw in the ground screw site on the metal box.  Both Romex grounds ground there.  (in metal boxes always ground the box first).
Get a pre-made metal jacketed wire whip, or make one out of AC cable and knockout fittings. If it has a ground wire, you got MC cable by mistake.  Must be &lt;6' long so the shell can provide ground.
Up in the box, do the expected thing.  Fit a blank junction box cover.
Down in the LED fixture, splice hot and neutral.  Ground is delivered via the cable shell and properly fit connector, so it doesn't take any space in the box.