Smelly Flip covers

I've been using flip diaper system on my son since he was born (currently 5 mos) and occassionally take breaks and use disposables. I have read post about the care of flips but my children are very allergic to 99% of all detergents including the free and clears tide being the worst, dreft makes me gag. but everytime my son pees the covers smell and wiping them down is not helping either. How am I suppose to keep the smell away and keep it on a budget? The whole reason I'm using cloth diapers. We're poor so we can't just go online and spend 30$ on some odor reducer spray that lasts about half a month. Is there something here at home or I can find anywhere? I use Purex free&clear, oxyclean and a half a cap of bleach in every wash. Oxyclean i know helps keep the wretched odor out of the inserts best but there is no winning with the covers. I've used borax too and that helps keep the softness best but the local stores don't always keep it stocked here. My only solution to the problem is just buy more flip diaper covers and not fret over the stinch.

I've been cloth diapering for over seven years, and I bet the problem is your detergent. The F&C are the worst of the worst. You need to also look up the hardness of your water, as minerals In the water can react with your detergent and leave a stinky residue. Dreft is terrible for cloth diapers too. As far as grocery brands, I had good luck with regular Tide and Planet. But I switched to Rockin Green and all the problems went away. Meanwhile, wash those covers with regular blue Dawn (google stripping cloth diapers) and start over, basically.

is it sunny where you live? Hanging them in the sun really helps!

Have you tried just taking the cover to the sink and washing it by hand with soap and water? dawn dish soap.
What about stripping them, have you stripped them yet?