To be honest, it is not the lack of olympic plates that is the problem, but the lack of flexibility in your body that is forcing your back to round out with your current set up.

If you can set up a sturdy block/platform to raise the bar, then thats fine, but make sure it is very stable, and the correct height. Another option id to widen your stance, adjust your grip, and work on sumo deadlifts as an alternative.

Outside of that, you should really look at improving your soleus, gluteus, and hamstring flexibility so that your back does not 'tuck under' no matter what ROM you are lifting through.
Don't let the door hit ya' where the good lord split ya'