bashing sleeper/couchette cars

Please help, friends. I would like to model the long distance overnight trains that travel across Europe, for example, Prague-Amsterdam, as such a train could plausibly connect the very ends of my layouts. I am furiously repainting the Czech, Dutch, and Austrian cars that might be needed in such a train. The German ones already exist. What is entirely missing, though, is a couchette or sleeper. Can anyone offer some suggestions about a Marklin base model to hack....and how to hack it, ie what I would need to do?

Most of the time the couchette cars looked like normal compartement cars. The biggest difference is that usually one end door is removed and the bathroom is enlarged to include a shower. Modifications could include filling in the door, removing its steps and painting its window white. These cars can be used as regular cars for shorter trips.

The sleepers usually have different windows, which is a little harder to make and generally requires certain window frames to be made smaller or filled in completly or new ones cut, but there are examples, where the sleepers look like ordinary 1st class compartement cars from the outside (except the decals) or 2nd class cars with every other window filled in.

ps: Maerklin made a few sleeper cars in the past, but they are older designs.

kvp, your suggestions fit with my observations. Now....are there some drawings? that I could work from? I'm thinking of one MU-type sleeper, and one couchette.

here's a quick update....drawings and the start of a Wlab MU-type....
web.me.com/ondrejmitas/tissuebox/trains_in_progress/trains_in_progress.html

Lots of progress. I have definitely gotten into the rhythm of chopping up marklin cars, filling windows, sanding, etc. Please see progress:

web.me.com/ondrejmitas/tissuebox/trains_in_progress/trains_in_progress.html

Andy, very nice progress. One day I'll be getting at this.