How do you articulate the uvular trill, when you can already articulate the uvular fricative?

Question

Key Assumptions: 1. My uncle speaks only General American English
(so he cannot resort to other languages' phonetic inventories).

2. Whenever he tries to phonate the uvular trill [ʀ], he fails and instead phonates perfectly the voiced uvular fricative [ʁ] (so about which I am not asking here).

3. Having perpetually rewatched this animation of the vocal tract for [ʀ] (see this), he knows perfectly the physical corporeal features of [ʀ], but which still fail to aid him to phonate [ʀ].
   So please do not repeat or explain with physical features.

4. The only solution appears to be this method from which I learned to phonate the alveolar trill, which cleverly exploits and combines phones in English to mimic another phone.

5. [Source:] My trick is say /a/. Then /g/. Then "gra". Those two sounds are made at the same position of the tongue and uvular.

Problem: Sadly, 5 did not aid my uncle.
So what other (combinations of) phones in English should be tried to phonate [ʀ]?