Bathtub shower door rail

I put in a shower door on our bathtub some time ago. Following the makers instructions, I attached the side rails by drilling through the tile on either side, then caulked the bottom rail to the bathtub.
Unfortunately, its the kids bathroom. It gets a lot of use, and that bottom rail gets flexed a lot. Twice it has separated from the caulking, leading to water seeping under the rail.
I believe the manufacturer's instructions to be flawed. The bottom rail appears to need more firm mechanical support to the bathtub. My thoughts have been: epoxy to the tub, or drill down into the tub through the rail and use self threading screws with sealant. The second scheme seems to be a bit more servicable.
Anyways, just wonder if anyone else had the same problem and what you did, and how that worked. Thanks.
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<< I believe the manufacturer's instructions to be flawed. The bottom rail appears to need more firm mechanical support to the bathtub. >>
Certainly makes sense. If you decide to go that route, use stainless steel screws. If the tub is cast iron, drilling and tapping a hole for SS screws isn't all that difficult. Self tapping screws are usually hardened steel and may not survive well in a bath environment, and in cast iron aren't a very happy combination. Plastic shims under the channel to prevent distortion from the screws would be good practice. HTH
Joe
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Seems like sound advice, thank you.
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