The OP I think seeks a "recipe".
First of all my Respironics went NUTS on cheap inverter power. The images in the display window looked like Sanscrit. No Way Jose, will I ever try connecting to modified sine wave inverter again. I do not want a broken BiPAP.
So I decided to go direct. 12 volts from the coach to the socket on the BiPAP
- I took the power brick cord along to use the plug as an example of what I needed to buy
- A laptop repair service is a perfect place to have the plugs installed on a custom length cord
- But you will need the other end of the cord opposite the plug that goes into the cPAP
- Don't buy junk and 99% of do it yourself plugs are junk
- See my recommendation for a great quality cigarette lighter plug below
- The laptop repair place will SOLDER both plugs onto your new custom length power cord
- It will not pull apart.
https://www.bluesea.com/products/1010/12_Volt_PlugLOOK in Amazon for this plug ---Blue Sea Systems 12 Volt Plug---
Now for the twin wire. not too big, not too small. Need very supple wire that is not glaringly obtrusive. Supple means it will be easy to wind up and will lay flat on the floor. Just in case you need to run wire to a distant cigarette lighter socket...
http://www.ebay.com/itm/SPEAKER-WIRE-50-FEET-18-Gauge-High-Quality-For-Car-Home-USA-SELLER-FAST-SHIP-/170879645576?hash=item27c9387788:m:myd8tew7JqMvoaEQnPhVv8QCarry everything into the laptop repair place and plunk it onto the counter. Being you have already measured out how long a wire is needed the rest of the work is in their hands, not yours.
"Just right size" means you can use this wire without or with the cPAP humidifier going. The price of the long length of this wire won't kill your budget.
Finally, when you go to pick up your assembled cable, take the cPAP along. Proof that it works right on the counter.
And yes the Blue Sea cigarette lighter plug is worth it. Lesser plugs are junk and you do not need to deal with a plug that fails.