the "wee" bottles that go under the

Hello

Inside the thetford toilet (c2, c3, c4) where you slide the cassette there are four small recesses for putting the wee / little aqua kem bottles for your pink, blue and green.

well I always get the big bottles (blue diamond, lidl bio or aqua kem) but i'm fed up lugging them about the place so would like to start storing the stuff under the cassette.

Can anone recommended cheap empty bottles that fit in there, just don't want to pay for more aqua kem bottles right now.

Cheers