This is going back a while, but I built a pulk a long time ago. I made it 6 1/2' long: just long enough that I could sleep in it by unloading it and setting up the skirting as a cover like a bivy bag. I used it half a dozen times before I sold it - not much snow on the coast

It pulled really well (skis) but I would make the next one narrower. Mine was about 30" wide.
The big difference between a pulk and the sleds you're talking about is that a pulk has a hull like a boat rather than runners like a sled, so it has a lot more flotation (and lower speed). I think that would make it more suitable for a double life as a trailer. The most efficient way to get what you want might be to have a collapsible towing dolly for it. You could build an A frame trailer and use hitch pins at the joints so that it collapses into a long, thin package that you can put in the back of the truck or on the roofracks.
I'm envisioning the folding bumper hitches used to tow a car behind an RV, but connected to an axle rather than a bumper...
Because of how narrow your rig would be, you could also use a normal trailer axle to match the track width of your tow vehicle.
The 'design' for the one I did came from the ones we used in the army. I spent a lot of winters schlepping one around the Chilcotin and Wainwright so I had plenty of time to think about how to build a better mouse trap...
Those ones were metal (either magnesium or aluminum) and the bottoms were pretty chewed up from dragging them over rocks and logs. Putting some runners on the bottom would help with that, but the extra surface area would create a lot more drag in snow.
I guess you could build it light and screw some plastic sheet (HDPE,UHMW, etc) to the bottom both to make it slick and to help shed road debris.
Food for thought: a lot of ski tourers make their own from those plastic kids' sleds.

An idea I've been kicking around:
I have an old snowboard and a couple sets of X/C skis I'll probably never use again (blown knee) so I've been thinking of a high-speed sled for grownups - sort of a GT SnoRacer that you lie down on. The snowboard will be the base and the skis will be cut down and canted/toed in for steering (like a snowmobile). It will probably be a frame of tubing and a bed of cloth or mesh to keep the weight down.
It's a 'someday' project though so I don't know when I'll get on it...