At Texas Tech their minimum model scale for the water tunnel was 1:24. That was for 'qualitative' work only. No measurements possible.
The underwater tow-tank used 2/3rds-scale models. $ 68,000 for a model, but the 6-axis sting provided meaningful quanta. SCUBA divers would be down there with video equipment to capture flow imaging.
AeroStealth purchased a $40.00, 1:24-scale die-cast Tesla Model X online and shipped it to me. I created a paint-matched boat-tail for it, and shipped it back for his You-Tube channel.
If you could find one of these for the IONIQ, it would end up cheaper and better in the long run. Cheapest $40 you ever spent.
As freebeard said, the HOT WHEELS is Reynolds number challenged.
If you know the rivers flow velocity, there are formulas and tables which would help you determine if you're achieving supercritical RN. If you're not, you're just gonna be peeing in the ocean and expecting to experience sea level rise.
