Haymura,
In case you cant locate a fabricator to make you an attachment, there are "Duet Tandem" rentals by a local non profit organisation. Detail and contact information is located at the bottom of the page
Reach Child and Youth Development Society - Reach Child and Youth Development Society
No affiliation just google'd around for you and that's the best I can provide.
I'm on a few cycling forums and I'll ask around also.
PS, good on you for taking your cousin out.