that polaris one isnt a true rigid box, its more of a stiff type foam. If its for a carbon road bike i think i’d go for a full on hard case.
bikeboxalan ones are very good
http://bikeboxalan.co.uk/
(suposed to be able to get a mtb in one as well, but pricey (although they hold prices really well from the 2nd hand ones i’ve seen). You can hire them from bikeboxalan. Check the dimensions though, i measured it and it wouldnt fit in my car. Supposed to be mega toughh though.
Polaris also do a full hard case one,
http://www.pod-it.co.uk/
i used that for my carbon road bike this year (hired one) and they are pretty good, but i think you’d struggle to get a mountain bike in one.
Problem with full hard cases and mountain bikes is the weight. A proper hard case is about 10k (some are more), so with mountain bike in you may be over the weight limit.