I am struggling to set the C0 of a weld?

I'm currently using :GetBoundingBox() to get an outer box around a moving model, a part known as testPart is CFramed to the values returned by the :GetBoundingBox() which also includes a size. But then I would like to weld said part to the model with it's current CFrame.

local testPart = Instance.new("Part", workspace) -- creating the outside box part
testPart.Transparency = 0.5
testPart.BrickColor = BrickColor.Green()
testPart.Anchored = false

local weldTest = Instance.new("ManualWeld", testPart) --creating a weld
weldTest.Part0 = testPart

local modelCFrame, modelSize = workspace.Model:GetBoundingBox()
testPart.Size = modelSize -- setting the part size to the model size
testPart.CFrame = modelCFrame -- setting the part CFrame to the model CFrame
weldTest.Part1 = workspace.Model.insidePart
weldTest.C0 = testPart.CFrame --now im struggling with this part

The last line is causing me problems as this is sets how the testPart is welded to a part that is within or outside the model. This inside part is already welded to the model itself.