When unequipped, what would I use to put a tool on a player's back?

This is what I use to do a similar thing. I put a sheathe on my player's back when they equip a katana.

local sheathe = yourSheathe:Clone()
sheathe.Parent = Character


local C = sheathe:GetChildren()
for i=1, #C do
     if C[i].className == "Part" or C[i].className == "MeshPart" then
        local W = Instance.new("Weld")
        W.Part0 = sheathe.Middle
        W.Part1 = C[i]
        local CJ = CFrame.new(sheathe.Middle.Position)
        local C0 = sheathe.Middle.CFrame:inverse()*CJ
        local C1 = C[i].CFrame:inverse()*CJ
        W.C0 = C0
        W.C1 = C1
        W.Parent = sheathe.Middle
    end
end

local Y = Instance.new("Weld")
Y.Part0 = playerCharacter["UpperTorso"]
Y.Part1 = sheathe.Middle
Y.C0 = CFrame.new(0,0,0.8)
Y.C0 = Y.C0 * CFrame.fromEulerAnglesXYZ(90,45,0)
Y.Parent = Y.Part0

local h = sheathe:GetChildren()

for i = 1, #h do
    h[i].Anchored = false
    h[i].CanCollide = false
end

You may have to play around with some of the CFrame values tho to get it work just right for your weapon.