With those sensors, it will false alarm constantly on a shed, I guarantee it, and then you will not trust it, and then you will remove it, and then when you are burgled, you won't know...
That said Y I just bought a metal shed from Amazon as a temporary store so we can get on with doing up the annexe. £100 4 x 6 full height, with a metal raised floor frame, not the usual sat-on-the-patio crap.
It's a 'warehouse' clearance product so it'll probably arrive with half the bits missing and need sending back, but until then I thought that was pretty good since they are selling the non-floor model for £250 odd still.
Even the door contacts are chinktat contract special. Oddly it's the bells that give these away from the outside, unlike most badge up jobs it seems they really were specially made for this crud. IIRC they supply a 99p grade piezo siren you have to drill to the wall using No.4 screws, they supply the cheapest speaker wire to connect it up via a terminal block, then you put the cover over the top. Yeah as I say pulled a few out, but couldn't reuse a single wire.