>Perhaps try to make the Danube the northern border of Austria,
Even the Czechs were not that interested in acquiring the 1/3 or so of Austria which is situated north of the Danube. There were hardly Czechs there, so Czechoslovakia would have to entirely depopulate 1/3 of Austrian territory and find even more Czechs to settle there, or put up with almost exclusively German-speaking areas again.
>allowing the Czechs to take lake, take their own bit from German, and allowing the newly communist Hungarians Burgenland?
So you would have Austria give up Waldviertel and Weinviertel to the Czechs, Burgenland to Hungary, and then take in people who have absolutely no attachment to Austria except speaking (roughly) the same language? And where the memory of being under Austria consists of "Sendlinger Mordweihnacht"?
OTOH you could describe it as Bavaria absorbing a strongly reduced rump Austria. Not the other way around.
The only reason any post-war Austrian government would accept this deal if the one proposing it have parked a few tanks in the Hofburg.