The batteries I found are 160 Ah LiFePO4 cells, and you would need 16 cells at $207.92 each. A 240 watt solar panel is $500, and the inverter is $500. This setup would give you 8,192 Wh of extra electricity, plus .5-1 kWh from the panel, totaling 8,592-9,192 Wh of extra electricity, giving you slightly more battery capacity than a LEAF. Although, this extra storage cannot be accessed by the car while in motion. The power can only be transferred to the car through the inverter, through the level 1 EVSE, and then to the onboard charger while you are parked. So, the question is, at the same price as a LEAF SV, would you attempt it? I probably would, because the i is easier to manage in heavy traffic because visibility is better. My circumstances at the moment is I drive 40 miles to work, 30 of it on the highway at 60-65 mph, sit for 10 hours in an unshaded parking lot, and drive home. There are no plugs that I can find, so having this setup would help, while also making a statement. Of course, you'd also need a wall charger for the second battery pack if you plan to use it everyday, like I would, so you can recharge the second pack overnight. You could also manage a window fan or something to keep your car cooler while parked, powered by the solar panel. And if anybody doesn't know, the LEAF SL does have a solar panel on it, but it is a little 5 watt panel on the back by the top brake light meant to maintain the 12 volt aux. battery, adding no range to the LEAF. Wasn't meant to.
So, doable? Yes. Practical? Most likely, unless you don't need the range extension or have a plug handy to charge from. Affordable? About the price difference between the LEAF and i. The LEAF and the i have about the same economy on the highway, with the i having the city advantage, so you'd probably get better range than the LEAF, given that you have enough time to deplete the secondary pack.