You've only had answers from American English speakers so far, and they were (not surprisingly) baffled by your suggestion that 'dle' and 'dow' could be confused. However, from a British perspective, your question isn't as strange as it sounds.
In fact, in the region where I live, native speakers DO say the ends of these words in exactly the same way. In many accents of London and the southeast of England, people pronounce the final 'dle' sound as 'ow' : so 'middle' sounds like 'middow' (rhyming with 'widow'), 'bottle' sounds like 'bottow', and so on.
This not a standard pronunciation, however, and I wouldn't recommend that you copy it. Do as the other members have explained, and make sure that you pronounce the consonant 'l' at the end of words like 'middle' and 'bottle':
'Window' ends in an open vowel sound 'o', as if the 'w' weren't there : WIN - DOE
'Middle' ends in a consonant sound 'l', with the second syllable pronounced as a weak schwa sound: MID - DUL
I hope that helps.