Pocket Spring mattress with depth of 8 inch max

Hello,
Thank you for providing such an informative site. I am looking for some help.
My Grandmother has a small double antique bed. She is looking to replace the mattress, however due to the construction of the bed frame, the maximum depth she can accommodate is 8 inches. She currently has a foam/latex mattress, but is finding it uncomfortable / too hot. She would ideally like to replace it with a decent pocket sprung mattress, but the depths of the ones on your website seem all to be greater than 8 inches.
Could you recommend any options? I have personally suggested that she buys a new frame, but she seems to be attached to the bed frame!
Many thanks
Sophie