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 S312II fronts, JBL Studio S10IIs mid and S8IIs rear.
Single JBL S38 center channel
Power Sound Audio V1800, 2 MFW-15's, DIY ported UM18-22
BenQ W7500 & Silverticket 110" matte white 16:9