Connecting Meshes to Character (Suggestions?)

I believe that the best option would be to have a block the size of the characters Torso that is welded to the backpack, and gets welded to the players Torso when they spawn or however you choose to do it.

Here is a script I made about a year ago to attach blocks to player's characters:

function Torso(char)
    local g = game.ServerStorage.Backpack:Clone();
    g.Parent = char;
    local C = g:GetChildren();
    for i=1, #C do
        if C[i]:IsA("BasePart")then
            local W = Instance.new("Weld");
            W.Part0 = g.Middle;
            W.Part1 = C[i];
            local CJ = CFrame.new(g.Middle.Position);
            local C0 = g.Middle.CFrame:inverse()*CJ;
            local C1 = C[i].CFrame:inverse()*CJ;
            W.C0 = C0;
            W.C1 = C1;
            W.Parent = g.Middle;
        end;
            local Y = Instance.new("Weld");
            Y.Part0 = char:FindFirstChild("Torso");
            Y.Part1 = g.Middle;
            Y.C0 = CFrame.new(0, 0, 0);
            Y.Parent = Y.Part0;
    end;

    local h = g:GetChildren();
    for i = 1, # h do
        if h[i]:IsA("BasePart") then
            h[i].Anchored = false;
            h[i].CanCollide = false;
        end;
    end;
end;

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(char)
        wait(2);
        Torso(char);
    end);
end);

This code will move the model based on the piece called Middle (Note: This piece is the one that is the size of the Torso). I hope this helped. If it did please accept this as the answer so that people know it has been solved.