1 Jamie: What myth are we busting today, Adam?
1 Adam: Time travel, Jamie!
2 Adam: I have here plans for a time machine which I downloaded off the Internet.
3 Jamie: I hope you didn't pay for those.
4 {scene change: Nigerian Finance Ministry}
4 Nigerian Finance Bureaucrat: Finance Minister! Someone bought our "time machine" plans!
4 Nigerian Finance Minister: I told you it would work!

I'm amazed there hasn't been a crossover between these two themes before now.

I was also shocked to discover that the characterstic speech bubble colours for Adam and Jamie match the ones for the Nigerians. I suspect that when I first did a Mythbusters strip I just copied the colours off the Nigerians.

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2020-03-21 Rerun commentary: I checked the Internet. I found surprisingly few downloadable plans for time machines!

Here are free downloadable plans for a "Time Machine". Though I suspect this one might be false advertising, and not actually be intended for travel through time.

Or if you want you can buy actual time machine plans in a book from Amazon. You might just want to read the reviews before spending your hard-earned cash, though.

Here's a filed US Patent for a time machine. (The patent was not granted. You can file a patent on anything as long as you pay the filing fee.)

There is a scholarly article: Vaidman, L. "A quantum time machine." Foundations of Physics, 21, 947–958 (1991). https://doi.org/10.1007/BF00733217, which is behind a research publication paywall. You can pay 34.95€ to download it if you wish (at least that's the price for me in Australia - some of you may have access for cheaper than that). The abstract sounds promising:

A novel description of quantum systems is employed for constructing a “time machine” capable of shifting in time the wave function of a quantum system. This device uses gravitational time dilations and a peculiar quantum interference effect due to preselection and postselection. In most trials this time machine fails to operate but when it does succeed it accomplishes tasks which no other machine can.

... but not promising enough for me to spend 34.95€.

There are, oddly, virtually no other scholarly papers on how to build a time machine...

Except this one: Gray, A. "A design for a quantum time machine." Quantum Physics, arXiv:quant-ph/9804008, 1998. Which has no content but the intriguing message: "This paper has been retracted, for obvious reasons." It looks like a secret shadow world government wants to keep the secret of time travel to itself, obviously.