You need no blower, simply leave some space between the components so that heat does not pile up and that you have a little airflow on the top and the bottom of all devices.
For RME devices I am using Thomann Eco compact racks with 23cm depths, for three devices I use a 4RU rack at minimum for that purpose. With 23cm the rack is not too depth so that you get a good airflow.
For the devices with more depth like e.g. a Lexicon PCM91 I am using a similar rack with 35 cm depth. This is planned for two devices, so here also 1RU more.
UFX+, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1650v4, Win10Pro22H2, Cub12Pro