Tar paper the roof of the shed. You have a tight space, so it will be more challenging.
These 3D printed tanks may work for you also.
The ladder is leaned up against it in the picture and would have to be glued. Tank bottom is 1 1/4 not including box off to side. Total height is 3 1/2 from bottom of tank to top of pipe when put together.
www.ebay.com
This modular chemical tank kit including the detail kit can be built in various heights up to 90mm. The parts for the chemical tank include 1 base ring, 4 center rings, 1 top ring with safety rails and 2 versions of the top cap.
www.ebay.com
I'm going to kit bash a Walthers Interstate fuel and oil dealer that I picked up for $15 into a smaller transition era industry.