I researched and compared travel insurances that include cover for backcountry ski touring for 28 days in Canada for a NZ-resident couple in their thirties. Most travel insurers exclude skiing outside of the ski area, or off-piste. These are the options I found. Most also offer annual options which are comparable to 4-week single trips.
DISCLAIMER: I am not qualified to give financial advice.
The new year (this post is a little late!) has brought me new opportunities and some new roles;