You could probably just go buy some LiFePO solar kit batteries from Amazon to power your homemade electric car. They are expensive but readily available right now. They have been getting cheaper in the last year though. A 12v 100Ah one can be as cheap as $200 ...

If you took about 30-40 of those 100Ah solar batteries and wired them in series, you could make the 350-400 volts that powers a Tesla and have a comparable but not as good range. 35 of them could be about $7000 to purchase at Amazon prices.

That's possibly not even feasible with the logistics of that many batteries, but that's an example of what's available on the consumer market and comparable to what Tesla is using to make their Model 3's power. I think it's something that could be achieved by a hobbyist with a good understanding of the electronics technology and access to a good workshop. In fact I've seen it done on TV when Anthony Keidis had an old muscle car converted to electric, well over a decade ago.