Is it just for a one night outage max? Is the battery sized right for your machine and settings? For extended power outage, you may reconfigure your CPAP, especially WRT Climateline tube and heated humidification. That can buy you a lot more run time on any given battery.
Assuming a 50 AH battery gets you where you want to be, it should work, but I don't think you really need the $65 battery box unless you just want it for asthetics (in which case, knock yourself out):
* The battery and a simple cigar lighter plug with alligator clips for $5 would get you there**
* The box is designed for a type 24 or 27 battery which is a lot bigger than the one spec'ed in the question - but much heavier too
* The 5 amp BatteryTender is big and bulky***; it is faster charging but one of the smaller wall wart models may be better suited for float maintenance***
** assuming Resmed's plug include a fuse - I assume they do - Respironics cord is fused
*** about the size of a pound of cheese; charge speed - to pick a number for a given charge state, charging times for your battery with the 5 A may be 4 hours, with a 1.5 A charger may be 12-15 hours but the maintenance function would be the same;