I often explain to students that if they misproduce prevocalic /r/ and substitute
/w/ for /r/ as in west for rest that there can be confusion with listeners. I use the EWR 8 deck set
prevocalic and medial/final /rl/ deck to visually show the difference between
rest as in your bed and west the direction.
I place the two cards side by side and visually show them the difference while working on auditory discrimination.
Howay for Wodney Wat is also a great book to use with students for showing definition confusion.