According to Pioneering Made Easy, a team who work with The Scout Association, you can build a cheap homemade raft with nine 25 litre plastic drums. However, their raft is intended for fairly large parties of scouts. Borrowing from their plan, you can make a smaller raft for fewer people with just three drums.
Lay out the two 2m staves and two of the 0.7m staves on the ground to form a rectangle. Lay the remaining two 0.7m staves in the rectangle, parallel to the other two and equally spaced along the rectangle. Check that the 0.7m staves are now 500cm apart.
Place the three plastic drums on their sides in the rectangular spaces formed by the staves. Move all the staves inwards slightly so they come into contact with the drums and overlap each other, with the shorter staves on top. Adjust the staves to make the overlaps roughly equal, for cosmetic reasons and balance.
Tie the staves together with sisal, using square lashings.
Fix the drums in place with two pieces of 2m long sisal each. Lash the end of a piece of sisal onto one of the drum’s neighbouring 0.7m long staves near one end of the drum. Wrap it around the drum tightly two or three times, then tie it to the other neighbouring stave. Follow the same procedure near the other end of the drum.
Tie in the remaining two drums in precisely the same way. Grab your oars and head for the water.
Use clean drums. Wear buoyancy aids when rafting.