Either carry for real or don't. Substituting a fake gun is just bizzare, and you're going to have just as hard a time - maybe even harder - trying to explain to a superior why you are trying to conceal a fake gun (as if "the real one is in the car" is going to elicit a satisfactory conclusion).
Yes, the decision to carry at work when prohibited from doing so is difficult. That is a personal decision, and I can respect either choice (frankly, the odds of getting fired and having no income are much higher than having to actually defend yourself, at least in my cube-farm environment). Either way you choose, take it seriously.
If you choose to carry at work, note what others said: while you may be hyper-aware of your gun and expect everyone be staring at you, fact is most people haven't a clue until the thing goes THUNK on the floor in front of them. Concealed means concealed, and if done right it won't be noticed.
If you choose to not carry at work, get suitable equipment: a Mitch Rosen Workman holster can pop on/off the belt trivially yet be secure, and a safe for the car can be obtained. If you have to fumble with it, you don't have the right equipment.
A rubber gun does nothing to protect you, complicates your thought process to no good end, and makes people think you're extremely, even dangerously, strange if they find out - I'd bet you'd get fired anyway. Don't play with toys when you're dealing with weapons. Make your choice - carry or don't - and do the best you can with either.