Per the 49 CFR (Hazmat regulations)
"Marking Specifications"
49 CFR 172.304 and 172.308
Markings must be:
Unobscured by labels or attachements, And (Continues)
And
"Motor Vehicles and rail cars":
49 CFR 172.516
In addition, each placard on a transport vehicle, bulk packaging, unit load device or freight container must be:
Located clear of any appurtenances and devices (I.E. Pipes/ladders)
(Continues)
All the above is pulled from the hazmat requlations. Basically you can't secure a placard with anything that will block part of the placard itself.
Personally I recommend a coat hanger that is bent out so the hanger is hidden by or right against the outside edge of the placard holder... Anything like wood or a hanger in the middle of the holder is considered obscured and is subject to a fine by the DOT...