1st of all congrats, an excellent choice. Don't forget a suitable USB3 stick for DURec/backup recordings if needed and an USB extension cable so that the stick can not break easily if somebody moves nearby/to close at the front of the UFX+. There is a sticky thread about tested USB3 sticks with good controllers which can write a lot of data sustained which is needed as DURec has no memory to buffer: https://forum.rme-audio.de/viewtopic.php?id=11257
For better heat dissipation I would use a 2RU (rack units) rack and leave some room under and over the UFX+. This room you can use to place an USB3 stick for DURec on an extension cable.
I am using these cases which are 23cm deep.
https://www.thomann.de/de/thon_rack_2he … ref=mobile
The higher the depth the more tricky to plug things at the back.
Currently I use a 4ru rack for 3 devices (UFX+, XTC and 12Mic). See here: http://www.tonstudio-forum.de/index.php … -Rack-JPG/
Narrower I wouldn't plan. So for 4 devices I would take 6 RU.
Should you plan to integrate a 2nd 19" unit into the rack in the future, then I would directly take a 3ru rack.
Whether you buy from the store or T. shouldn't matter much, I mentioned my rack because it is more compact. With the 23cm it's easier to plug devices at the backside.
Should you plan to use more quality reverb units from Lexicon (PCM8/9*) then be careful, they have more depth than 23 or 25cm. I would put devices with much higher depth into a different case, so that cabling is easier as mentioned.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13