Advice on Insulating sloping ceiling upstairs

The sloping bits are also called 'skielings' and there are some more threads on the problem on the forum.
I too am trying to think of way to tackle this. Older houses with lath and plaster have the additional problem of a very rough surface so fluffy stuff won't slide in. I have considered two sheets of plastic of appropriate width to sandwich the fluff to insert it over the plaster, then slide out. A complication is that i doubt you can buy 2 in fluff anywhere now and would have to split 4 in to leave the air gap. However mostly for me the gap at the top is blocked by ends of the cieling laths.
One advantage of lath & plaster is the wood content to provide a bit of insulation (wrt plasterboard) and the sarking (wood under tiles) helps too.

Recent ideas (please comment) include:
1) locate all noggins and map out the 'compartments' between the wood structure. Drill a 2 in dia hole in the middle top of each section, and small holes in each corner. Thread strings from each corner to the big hole. Make a netlon or sacking bag that fits the compartment, insert through big hole using the strings attached to each corner topull it into place.
Fill the bag with loose fill beads or some such. It will need internal strings to stop it filling out >2 in rather like the buttons on a dimpled cushion.

2) Current favourite. Cut 2in x 14 wide slot right across the top of each compartment.
Prepare 1 in celotex board the sixe of the compartment and make it flexible by cutting lines across it just leaving the paper on one side to hold it together. Once the first one is in place it can be pushed up and a second put in. It would help to be able to glue the two together once in and pull them down onto the plaster. Somehow get a bit of fluff between plaster and celotex to help prevent convection up the inside layer.