Inspiration Healthcare had a humidifier they were marketing a few years ago specifically for use in transport ventilation.
I discounted it at the time due to the manner in which they were suggesting that power be applied to it, therefore we never did get round to testing it at that time, but I believe they have overcome that problem since.
Might now be worth a look.
Regards,
Stephen
Any thoughts and opinions are purely personal & not representative of any prior, current or future employers. Any resemblance to persons living, dead or fictitious is just shear bad luck