Greetings,
Firstly, I am not sure if a sleigh pattern like this would be an option but, hope it may help.
https://www.bing.com/images/search?view=detailV2&ccid=3wmG5Fvq&id=35E4907E5790453DF720C4F174CA25E2E8B15CA3&thid=OIP.3wmG5Fvq3RZqIYch7rhnoQHaEq&mediaurl=http%3a%2f%2fwww.scrollsawvillage.com%2fuploads%2fgallery%2fcategory_3%2fgallery_2254_3_5181.jpg&exph=252&expw=400&q=Sleigh+Patterns&simid=607990887189647054&selectedIndex=10&ajaxhist=0
Secondly, I think if you take a picture of the end view (front or back) of the sleigh on one side of your wood stock, you would just have to cut a notch up to the bottom of the sleigh and
the side profiles.
Then, you would need a photo of the side of the sleigh oriented so the runners are on the bottom where the notch is already cut and then tape the pieces of
stock back together for stability while you cut the side profile. The down-side to this cutting would be the top of the sleigh is still all one piece instead of
hollowed out where the passengers would sit.
Finally, to try to answer your question more specifically about HOW to make such a pattern, if you look at some of the images here of 3d or Compound cut such as the
reindeer and the dolphin. The reindeer shows the end view (compare to the end view of the sleigh) and the side view (as compared to the side view of the sleigh.
The dolphin, however, shows a top view of the dolphin and the side view of the dolphin to get the desired effect. https://www.bing.com/images/search?q=compound+cut+reindeer+plans&id=1B806BF0A652657183CC2E5B65D2F382254101A9&FORM=IQFRBA
I hope this makes a little bit of sense and helps you out. I'll try to answer any questions to clear up any info I did not explain well.
God Bless! Spirithorse