I don't know how much a plastic barrel costs but I've been using a Seal line pack with a plastic trash can inside for a rigid waterproof pack. It's pretty cheap, especially if you already have the pack. The trash can can't be more than 10 bucks. I had to trim mine down to the proper height to make it fit.
				
			 
	 
 
		 
			 
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		