Assuming you don't hit traffic...

Long Beach Airport, Long Beach, CA: 36.2 mi, 46 mins

Route: I-605 N and I-10 E

John Wayne Airport, Santa Ana, CA: 34.4 mi, 41 mins

Route: CA-57 N

LAX, Los Angeles, CA: 45.5 mi, 51 mins

Route: I-105 E, I-605 N and I-10 E

Unfortunately, yeah they will all relatively be a long drive. These are the closest airports to the convention. Just see if you can get into John Wayne or Long Beach from Indiana and you guys will be money. No problem, by the way...