A one day MR to get 8K miles? I don't think that's possible.

The one I could think of is MIA-LAX-ANC-ORD-MIA but you would be leaving in the late afternoon of day 1 getting to ANC(via LAX) in early morning of day 2 and finally doing a red-eye through ORD on day 2 and returning to MIA in late morning on day 3. The total cost is ~$550 all in and it gives you just over 8.7K miles.

The best part? it's all domestic, so no need to mess around with immigration/customs.

The worst part? The LAX-ANC-ORD flights are on AS, so economy is it, unless you are willing to purchase upgrades (if available) but obviously that would bring the price up another few hundred $.