Tarptent Sidecar. From website: “Adds a fully enclosed sleeping compartment for the StratoSpire series vestibules or can be pitched separately. Fits children up to about age 12 or any dog.”

I set this up for use on BP trips but my dog refused to get in so it’s in like-new condition, except for the directions which are a bit dirty but readable.

$50 incl shipping. Paypal. Add 3% for GS if desired.

(Featured image from Tarptent website.)