After poking around online, I found a few different ideas to make a ski kit that would work behind my fat bike. The options available from Thule/Chariot were expensive and only sold as a kit for XC skiing, and I only needed the ski portions since the whole waist belt system wouldn't really work on a bike.
I also wanted an easy on/off option so I could ride to the trails with the wheels on, then attach the skis. This is what I came up with.
Step 1: Splitting the Conduit.
First I found some PVC electrical conduit with a 10" radius bend, which matches pretty perfectly with the 20" diameter wheels on most bike trailers. I think they were about $2.98 per piece at Lowes.
I used a short blade on my jigsaw so I could cut each side of the conduit, making a tray for the wheel to sit in. Once split, I sanded down any sharp edges with a sanding block.
A side note on the conduit: while at Ace Hardware picking up some bungee cords, I saw some very similar conduit without the bell end, which might have been marginally better for this application due to a snugger fit at the end.
Step 2: Mounting the Skis
After removing and discarding the old ski bindings, I drilled a pilot hole and attached the conduit to the skis with a short wood screw. Had to be careful to try and center up the skis so they will track straight.
FYI. the stock skis on the actual Thule Chariot ski attachment are 110cm. I found a 120cm kids XC ski at Play it Again Sports for under $20. They're very lightweight and have a wood core.
Step 3: Attaching, Continued
I originally bought bolts and T-nuts to attach the conduit, but after the single small wood screw found good purchase in the core of the ski, I opted for a quicker and easier option. Two longer wood screws went right through the skis, following pilot holes I drilled so the conduit wouldn't crack or split.
Step 4: Trimming the Screws
Obviously having the right length screw would have been better, but some quick Dremel action solved that issue pretty quickly. I might clean this up a bit more later, but I don't think it will have much affect.
Step 5: Attaching to the Tires
Another quick and easy option for now, I used 10" bungee cords ($4.99 for a 4 pack at Ace Hardware) to attach the conduit to the tires. I originally envisioned a velcro strap that would stay attached to the conduit, but couldn't find exactly what I was looking for. Might make something like that later since the bungee cords are a little tricky to get on/off with gloves on in the cold.
Step 6: Test Ride
This was the easy part (aside from the extra effort of hauling a trailer)!
The whole unit functioned pretty well the first time out. I'm definitely going to add one more bungee cord to each wheel, right where it sits in the bottom of the conduit. One one big bump the wheel popped out sideways just a little bit. Eventually I'll switch to velcro straps once I have time to sew them up. For an initial effort, this worked really well and the kiddo was stoked to come along on a fat bike adventure.
Step 7: Keep the Kiddo Stoked!
Reward kid with hot chocolate, pancakes or waffles! I may toss a hot water bottle in with him next time, since I ended up having to take off the wind screen (it fogged up and he couldn't see). He still had the bug screen to protect him from snow, but the wind made him a bit colder when I was going downhill. He LOVED going downhill… which made for lots of uphill runs as well, adding a great workout for me. Good to remember that even though you're probably overheating, the kiddo is just sitting there and might be colder.