Cut off hard part from top and bottom of Kobocha then cut into 3 to 4 wedges. Scoop out the seeds and cut into chunks.
Steam in steamer, covered, for 15 minutes over med heat then use a chopstick and see if it can be pushed into center easily. Remove and leave covered for 5 minutes.
Remove skin. Mash flesh and push thru sieve or process in food processor. Transfer to mixing bowl and add rest of ingredients. Mix well.
Roll out makisu sushi mat as if you were making sushi. Wet muslin slighly and lay on mat. Spread mix on evenly. Hold nearest end and tightly roll up to other end. Close both other ends by rolling up or folding over muslin.
Put roll in steamer for 5 minutes. Remove from heat and let rest 5 more minutes.
Open the makisu when roll has cooled. Cut into 1 inch thick slices and serve cold on plates with thinly sliced nashi and kaki if desired.