Hi, Claus.
I've been using a similar idea for a couple of years now quite successfully. The main differences are as follows:-
1.My cockpit is shorter than the BR's so I do not need the boom and yard as a ridge pole. Instead, I have the forward edge of the tarp bowsed down tight over the edge of the spray hood, using 3mm braid instead of shock cord to tie the corners to small bull noses screwed to the rubbing strip. Braced against the elasticity of the sprayhood frame it makes a gale proof seal, even on a mooring, head to wind.
2.At the ridge of the “tent”, about 20 cm from the mizzen mast, I have another length of braid tied as a noose over a golf ball pushed into the tarp from the inside. The ball gives a large surface to spread the load as this braid is tensioned hard and tied around the mizzen about 10 cm above the top of the ridge. The remaining length of the tarp can be fastened around the mizzen mast for a weather proof seal, or, more often, left open for ventilation. The exact position of the golf ball can be moved back to have some of the cockpit uncovered if desired. Not a good idea in rain, as water runs down the side decks into the covered area.
3.The sides of the “tent” overlap the gunnel, nearly reaching the rubbing strip, to which they are attached in a couple of places with string and plastic bull noses, all easily done from inside the cockpit when afloat or, with the exception of the golf ball, from ground level when on the road and using the boat as a caravan. (Note: When towing the boat, unballasted, and making an overnight stop I have to keep it hitched to the car, otherwise, with 12 stone - ok , 13 stone - in the cockpit, the whole boat tips up, upsetting both stove and crew!)
4.Having the tarp outside the gunnels does not look as smart as your set-up. Less like a cockpit tent and more like someone has just thrown a tarp over the boat ( Doh!) but it has the advantage that rain on the outside and condensation on the inside are both directed over the side keeping all inside dry. I'll perhaps post some photos when the weather gets better – but, as a tent it is functional rather than pleasing on the eye!