On my road bikes with below the BB cable guides, I always add a second hole under the rear of the guide. Then file a groove in the guide to create a channel.

This makes a drain, while effectively prevents splashed water from entering.

However, if for any reason you prefer not to drill, you can attach a guide with glue or double sticky tape since the cable tension helps keep it in place anyway.

OTOH, if the frame wants the RD cable routed above the chainstay, you can simply route the FD cable under the BB without a guide. It will not move because it can't.