You’re right, there are multiple types of CPAP mask. I was writing about full face ones like these. In the cheapo ones we stock on my ambulance the seal around the mask is a silicone-ish tubing that clings to the skin very well.

For CPAP or BiPAP to be effective (and not piss off your ventilator), the circuit needs to be leak free. I’ve had full conversations with people on BiPAP before without my vent throwing any alarms. They have an expiratory valve so you can breathe out, but no outside air gets in. You’re fed solely by the air getting pumped out of the vent. These masks would be very easy to set up for pure nitrogen by using trach mask tubing (the ports are the same size) and a tank of nitrogen set up to an O2 regulator. Ditch the dilutor for one that’s closed off. Couldn’t tell you what they’re called but I’ve used them before and know they exist. This whole setup could be had for less than $50 if you’re purchasing wholesale