If for 2 years I worked about 30 hours off the books after my 40 hours, can I get the overtime that’s owed to me?

The law (Fair Labor Standards Act) requires a non-exempt (not exempt from overtime, like any employee paid on an hourly basis) worker to be paid overtime whenever he/she works more than 40 hours in a week. So at a basic level, you have a viable legal claim for the money.
Evidence of working does not have to be time cards: it can be credible (believable) testimony of yourself or other witnesses (e.g. co-workers), for example; videographic evidence showing you onsite is also valid and useful.
The issue you may have, however, is whether you were authorized to work: the employer, not employee, controls when employees work, and employees cannot obligate the employer to pay them (including overtime) when the employee was not supposed to work. 
Authorization can be implicit. IF the employer can be shown to have been aware that you were "sneaking in" and working extra hours, they condoned or approved it, even if in not so many words, and would have to pay you. "Letting" you work extra hours, when they knew you were doing this, if approving. On the other hand, if you hid your activities so effectively that they did not know you were working the extra hours, then they don't have to pay you--you can't force them to pay for work they did not authorize, even just by allowing it to go one when they were aware of it, you to do.
So the big hurdle, if you were "sneaking in," would be showing that the employer knew what you were doing and so effectively authorized it. If you think you can do that, it's definitely worth pursuing this for $57,000.00.