This wasn't too difficult once I'd found the easier speed zones.
Take an Ultima GTR 720 (for those without access to the 1020) and look through the shared tunes under Upgrades > Load Upgrade until you find a high-end S2 with a 10 in speed - a bit extra handling wouldn't go amiss as well, since the zones invariably have a few twists and turns.
Edit #1 -
Nosi90 has noted that the Ferrari FXX may be a better choice, although it is somewhat more expensive at 2.7M credits vs the Ultima's 150,000Cr. If you have it, or can justify the extra expense, give it a whirl.
Edit #2 - Thanks to
Groudon199 (and
https://www.trueachievements.com/X+Mr+Plankton+X.htm for the independent confirmation) for advising that repeating the same speed zone works so, if you find one that's easy for you, just repeat it twice more to complete the challenge. Personally, I found the one due South of Surfer's Paradise easiest. Can't believe I didn't check that one myself...
Alternate speed zones (and the ones I got this with) are two to the South-West of the Outback in the image below:
The one in the Outback furthest South is tricky, but IIRC you only need to hit around 90mph for 3 stars.
Don't forget you can use rewinds, even mid-zone, to ensure you hit the apexes and maintain your speed as best as possible.