Totally agree with signbrad.
In addition, a cover such that is being requested is generally temporary, as also mentioned when businesses are changing names or a new business is coming in. Perhaps, when you apply for your permits, your codes people will ask....... what period of time is this to be used ?? You say, as long as possible. They will most likely tell you that it's not legal, as the sign was never made with this type of installation in mind. It breaks electrical codes and wind load tests.
Most covers are put directly over existing faces, not an open cabinet. Wind will tear that thing apart
Also, when doing a bag sign, the sign is to be turned off to prevent various problems from overheating to melting. You'll be covering up the breather vents and most of the normally open ends and whatnot of the cabinets corners and retainers. Those covers are meant to protect a sign from penetration of all kinds which means it won't let most things out, either.
Get to know your business better before promising people things they can't/shouldn't have.