Though more costly, I think the respirator mask is more full proof and you can do it lying down.
I've tested it and you'll need a minimum flow of 80cfh. My 20cf cylinder lasted 15 mins at this flow. I might get a 40cf to be on the safe side.
Take a deep breath then secure mask and lay down. Exhale fully and wait a couple of seconds before inhaling. This will allow all CO2 to be purged from the mask.
Amazon product ASIN B00E4ZJI1E
These are all the fittings you need for a 1/4 ID tubing
Amazon product ASIN B008E5CUFG
Amazon product ASIN B004QAM166
Amazon product ASIN B013VQLSGO