A teacher of mine once told a classmate to practise saying 'kedentebedood' faster and faster until it turned into 'krentenbrood' with two rolling r's.
It's a dutch word meaning raisin bread, but I guess it could work for non dutch speakers too. The first, the third and the fourth 'e' are a neutral vowel which we used to call 'shwa' at uni. I'm not sure if you can say that in english too.
The second 'e' is more like the spanish 'e' and the 'dood'/'brood' at the end sounds like 'dote'/'brote'.
This worked for my friend, so you might wanna give it a try...
Good luck anyway