Try and find a CF Sterling SMG sling. Goes long enough to have the toy shotgun out of the way behind your back but easy to get at. Attaches with normal commercial shotgun sling swivels. Been using one on a non-toy, 18.375"(now 20", boring story) 870 for years.
Plan 'B' would be a 1" nyon strap with nylon or plastic hardware. All of which can be had in a discount fabric shop for cheap. If there's such a thing as a No Frill's supermarket in BC, they sell cheap ratchet tie downs with a 15' 1" nylon strap for a buck. Don't think you can buy 15" of nylon strap for a buck.