When it was fitted was the bottom suitably sealed / treated? Particularly if it was adjusted to fit. Do you have a weather bar fitted on the outside? You should be able to seal the dog flap with a suitable silicone sealant.

We stayed in a hotel a few days ago with a shower screen with a flapper, like the one in your picture. I put a bath mat by the opening and that a set red the splashes. I found it awkward as the flapper only opened to 90°, leaving a narrow “corridor” to get out of. I usually dry in the shower, putting a dry foot at a time outside, but the flapper position made this impossible.

The handles will be no problem. Do they have a sprung return? You can get fixed spindles like these - Zoo Hardware Taylor Spindle | More Handles - that will keep the unused handle rigid, this is useful if the handles rely on the spring in the catch to return them to their normal positin but otherwise it is personal preference. Something like this - Zoo Hardware Rebate Kit to Suit Contract Tubular Latch (morehandles.co.uk) - is required for the catch. It keeps the latch far enough out to meet the catch plate on the other door. If both doors open you will also need bolts on the door without the catch but how you do this depends on the arangement of the frame and of the doors. When I did it I used something like these - Eurospec Stainless Steel Lever Action Flush Bolts (morehandles.co.uk) - on the edge of the doors but a normal bolt on the face would work just as well. Andrew

You need to fit a deflector panel to the shower screen