Dinghy Mate by Moeller Marine. There are some gigantic design leaps or just sensible fine tuning. This idea is a fine tune, taking a portable fuel tank and shaping it to fit a small outboard dinghy combination. By fitting under the outboard motor it is safe and uses space economically.

$83 for the 3 gallon tank, and $120 for the 6 gallon version.
Moeller Marine’s Description
Portable Tanks
DinghyMate
Moeller and Scepter Team Up to Bring You the Most Complete Line of Portable Topside Fuel Tanks
3 Gallon Dinghy Mate & 6 Gallon Dinghy Mate
Engineered as a transom mounted marine fuel tank. Manufactured from lightweight, high strength cross-linked polyethylene for durable performance. Designed for simple mounting on the transom under the outboard motor of your inflatable boat or dinghy. Approved for use in Japan, Austrailia, and New Zealand. ISO & CSA certified. Patented design. Product # 04156
Product Dimension: 23.00 x 6.50 x 12.75 3gallon
26.00″Lx8.00″Wx14.75″H for 6 gallon