I've looked for the same a while back and found a few but the problem i found was that, if you use a 1/2 U panel, it offsets the piece of gear you place after it in a way that doesn't align with the rail screw holes making it impossible to screw in.
If you want your rack to not have any gaps then 1/2 U panels won't work. You would need 1/3 U and, depending on how you setup your rack, you would need at least 3 to compensate for the offset. The reason for this is that the holes on rack rails aren't all equidistant, they are in groups of 3 and between each group of 3 there's a shorter distance than the distance between each 3 holes.
If your rack is in an upright position and you don't mind not having the gear screwed in, you could get away using 1/2U panels and just leaving the gear unscrewed.