How can I get the area between multiple parts?

I have tried quite a few methods, I am trying to get the area between multiple parts so the item I place down can only be placed n that area, and not on the outside of that area.

Here is an image of the area: pasteboard.co/HVrdQ2a.png

local m = {}

-- Priority Variables
local uis = game:GetService("UserInputService")
local runService = game:GetService("RunService")
local rs = game:GetService("ReplicatedStorage")
local furniture = rs["Furniture"]
local remotes = rs["Remotes"]
local confrimPlacement = remotes["ConfirmPlacement"]

-- Secondary Variables
local studio = workspace:WaitForChild("Harbour Studio")
local studioFurniture = studio["Furniture"]
local owner = studio["Owner"]

--Model Placement Varibles
local plr
local mouse
local itemsFrame
local finished
local node
local preview
local pp
local selection
local canPlace
local rot = 0
local placing



function previewItem()
    if (placing == false) then
        return
    end

    local allNodes = studio["Nodes"]:GetChildren()
    for _,n in next, allNodes do
        node = n
    end

        if (owner.Value == plr.Name) and (pp.Position - node.Size).magnitude < (node.Position - pp.Size).magnitude then
            selection.Color3 = Color3.fromRGB(0,155,0)
            canPlace = true
        else
            selection.Color3 = Color3.fromRGB(155,0,0)
            canPlace = false
        end
        local angle = CFrame.fromEulerAnglesXYZ(0,math.rad(rot),0)
        local vec = Vector3.new(0,preview.PrimaryPart.Size.Y/2,0)
        local cf = CFrame.new(mouse.Hit.p+vec)*angle
        preview:SetPrimaryPartCFrame(cf)
end



function m.start(p, modelName, frame, fin)
    plr = p
    itemsFrame = frame
    finished = fin
    itemsFrame.Visible = false
    finished.Visible = false
    mouse = plr:GetMouse()
    --node = studio["Node1"]

    preview = furniture[modelName]:Clone()
    preview.Parent = workspace
    pp = preview.PrimaryPart
    selection = Instance.new("SelectionBox", workspace)
    selection.LineThickness = 0.05
    selection.Adornee = pp
    mouse.TargetFilter = preview
    for i,p in next, preview:GetChildren() do
        if not (p.Name == "Hitbox") then
            p.Transparency = 0.5
            p.CanCollide = false
        end
    end
    placing = true
    runService.RenderStepped:Connect(previewItem)
end

function m.cancel()
    if (preview) then
        placing = false
        preview:Destroy()
        selection:Destroy()
        plr = nil
        mouse = nil
        node = nil
        mag = nil
        preview = nil
        pp = nil
        selection = nil
        canPlace = nil
        rot = 0
        itemsFrame.Visible = true
        itemsFrame = nil
        finished.Visible = true
        finished = nil
    end
end

function m.rotate()
    if (preview) then
        rot = rot +1
        preview:SetPrimaryPartCFrame(preview.PrimaryPart.CFrame*CFrame.fromEulerAnglesXYZ(0,math.rad(rot),0))
    end
end


function m.place()
    if (preview) and (placing == true) and (canPlace == true) then
        placing = false
        confrimPlacement:FireServer(preview.Name, preview:GetPrimaryPartCFrame())
    end
end

return m