I have a slot ported um18-22 in a small enclosure and haven't had any issues with it yet. it's around 5.5 or 6cf. 17hz to 80hz flat within five decibels according to my REW measurements. it isn't impossible. slot ported is the best way to go to prevent lack of airflow. small isn't probably ideal since it does better in a big box but it's not impossible in something smaller. it just doesn't get fully utilized I'm guessing to its full potential.
Pioneer Elite SC-05
JBL Studio speakers
Power Sound Audio V1800, 2 MFW-15's, DIY ported UM18-22, SVS PB10-NSD
BenQ W7500 & Silverticket 110" matte white 16:9
Benq HT2050 2nd system-pvc foam board screen