You can't make it mandatory at the same time as restricting access to it. Voter participation in the US being low is a design feature. Having it on a Sunday, auto enrolling citizens, having more accessible polling places, etc would also increase participation.

I think mandatory voting is only reasonable if their is an official spoil option that should trigger something if selected enough. The outcome is tricky in parliamentary/party based systems, as if you have a revote just going to end up with the same parties, but I also think its quite undemocratic to force someone to participate and therefore tacitly support a system that may be screwing them. In a democracy no one should be forced to pick between (what they consider) terrible options.