Proposal one: After new borders are established make it clear that _any_ violation of those borders is tantamount to declaring war against NATO/the US (whichever is scarier).
This works.
However, something makes me doubt that any American leader would we willing to enter into such an entanglement.
Taiwan is infinitely more important to the United States, than Ukraine, and yet the diplomatic framework of the past 40 years has been that of "strategic ambiguity".
Before that, the United States simply decided that it was more important to develop relations with Communist China, and revoked its recognition of Taiwan and moved its embassy from Taipei to Beijing.
Then, in the 1980s, it was leaked that Taiwan had a secret nuclear program - like Israel and Apartheid South Africa. The United States promptly came in and through use of ultimatums got Taiwan to shut the program down. Basically - an earlier version of the Budapest Memorandum.
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Given that a Chinese conquest of Taiwan would actually spell very serious economic troubles for the US (unlike the fall of Ukraine), and yet the US still refuses to provide overt security guarantees - tells me that there's a snowball's chance in hell that the US will underwrite Ukrainian security.