Topper Dinghy on the Roof

[pete&gill]

Does anyone have experience of carrying a topper dinghy (hull 3.4 m long, weight 43kg) on the roof? Looks to me as if it should be possible to get one up there maybe using steps or an ally work platform?
Any advice on technique, best ladder bars (roller on the back one?) etc would be much appreciated.
Pete & Gill