Anything without cracks for mites to hide in would be an advantage - you can achieve this with a wooden house by coating thoroughly in gloss paint, but then you've got to set the time it takes to do that against the cost of an eglu . . .not sure what a plastic kennel looks like, sounds interesting though, might look into it myself if I increase my flock.
I built a house/coop in 'ark' style using some bought wood & some from Freecycle- spent about £30 on materials. I added a 1.5 sq m home-made rabbit run to this that I got from the ever-useful Freecycle, so each hen has over 1 sq m space. I painted it with wilko outdoor wood paint, trying to fill in the cracks, and gave it all a thorough dusting with mite powder before use- the powder lasts for several months per treatment.