"The following items are available for sale at most United Airlines airport ticket counters:"
"Bicycle box 69 in. 8.5 in. 33.5 in. $25.00"
Not sure if they'd sell to a customer of a different airline, but at $25 it must be a money maker and worth a try if your airline doesn't have a box.
In the past I've always found boxes or bags at Boston's Logan, Montreal's Mirabelle and Dorval, Calgary, Geneva, Paris' DeGaulle, Washington's Dulles, Bozeman MT, Missoula MT, Helena MT, Portland OR, Portland ME, Denver, LAX and Christchurch NZ. Not sure if the availability is still as widespread as some of those flights were 25 years ago on various airlines. None ever on WestJet so it would be wise for the OP to check in advance.
I've also used UPS for returns trips recently but after paying $170 last year from VA to NH because my box was 2" longer than a previous shipment + $50 to a bike shop for prep..... I may go back looking for $50 airline deals! The only time I flew SW was from St. Louis-Manchester NH without the bike.
PS Enjoy your Idaho trip! I'll be following your Crazy Guy journal.