Hi, Jeff Curtis.
You say you have had problems rigging a cockpit tent on Rhiannon - surely the best looking BR of the fleet - so you may well have tried and rejected my down and dirty version. It is not very elegant but it is quick to rig and stow (stuffed into a carrier bag on the Four Sisters, I fear), cheap to buy and effective in the wettest weather. It makes the whole cockpit of the CBL into dry living space - but it is noticeably warmer in the cabin!
You need a (roughly) 4m x 4m tarp, some 4mm string (non-stretch and UV proof)a handful of plastic bull noses + screws ..... and a golf ball.
Drape the tarp over the spray hood and pull down tight each side using non-stretch string through the corner eyelets to bull noses fixed to the rubbing strip. Dont be tempted to use shock cord, the elasticity of the sprayhood frame will hold everything tight. Any more give and the tent will flap and allow drafts.
Pull the tarp aft towards the mizzen and use the golfball, pushed into a sort of pocket from the inside of the tent and held with a noose of some sort. This spreads the load on the tarp and prevents tearing. take the other end of the noose and fix it to the mizzen mast at an appropriate height with a couple of round turns and half hitches. If you roll the sail around the mast, use flat tape instead of string to avoid damaging the sail. Now use string and bull noses to hold down the edges of the tarp OUTSIDE the gunnels so that rain or condensation from cooking doesnt wet the side decks. Pull down tight and the elasticity of the sprayhood and mast will keep it all as taught as a drum skin. There should be plenty of spare tarp on the mast side of the golf ball hitch to close off the rear of the tent if you wish. You wont need to if anchored - the tent is as good as a riding sail and no rain will come through the back!
