I constructed this for about $20!
10- 1"- 1/4" bolts
10- 1/4" lock nuts
20 - Washers
2- 55 gal plastic barrels (be cautios of former contents. find food grade, or soap)
Mounting hardware: I used a spare swing, and lag bolts and washers.
1/4' drill bit
ratchet and wrench
Step 1: Barrel Prep
Use the jigsaw to carefully cut along the inside rim.
Clean up the edge with sand drum on a drill or dremel, or an angle grinder .
Repeat with the second barrel.
Sorry for the lack of picture on this step, I did this some time ago.
Next do the same to the bottom. Be sure to leave about 1" around the edge to bolt together.
I used a screw and a sting as a compass from center.
Sand these edges very well so kids don't get scratched.
Step 2: Barrel Attachment
Now the fun part, (hope your not claustrophobic) Crawl in and and bolt those holes together .
Then drill and bolt the rest of the holes.
Step 3: Mounting
Next build a truss to hang it on. I went heavy duty with double 2x6 laminated with hangers to double 2x6 headers. I also boxed in both ends of the barrel end and lag bolted them to the tower. I used a spare swing to add support to the center.
I've had large kids in it without even bending.