Storing five gallon, plastic buckets, because they have so many uses, is great, until you try to un-store one (pull one from the stack). Then you are reminded of the power of a vacuum, and how hard it is to overcome one when you are the only one in the room. In the end, you have to call three friends and uncle Tim (the Toolman type) to get one or two free.
Generally, you wrap your feet around the bottom bucket and it works, but that REALLY is a slippery slope. It, as often as not, doesn't go as well as planed and hoped. Likely as not, you manage to turn a stack of buckets into two stacks of buckets.
To overcome the vacuum problem, I've wondered why bucket manufacturers didn't add a straw down the side of the bucket. Actually, several, in case the one got crushed. That would allow air into the bucket and solve the super vacuum problem.
Meanwhile, lacking the drive and equipment to add those straws, I experimented. Like Edison and other great minds (come on, just go with it), I learned ways not to do it. For example, hot glue.
A hot glue line ran down the side of the bucket does create a gap between the bucket it's placed on and the bucket it's dropped into, but hot glue could, I suspect, be used to provide excellent tire traction of an icy road. In short, it solves the vacuum problem and creates a friction problem.
Just one more experiment in, I borrowed my first idea, which was, cut 1/16th inch pieces off a board and just stand a couple of them along the insides of a bucket, before dropping the next bucket in. This worked. However, it meant I had to keep the thin sticks upright, as I dropped the next bucket in, and not lose track of them, for later re-use, when I pulled a bucket out.
Then came my stroke of lame genius - hot glue the sticks in place, so they could be stacked, then removed from the stack with relative ease until their end of use date. The stick part was solid. The lame part came with [and, please, keep this on the down-low], I actually considered hot gluing the sticks to the inside of the buckets, before realizing they'd work just as well attached to the exteriors, and wouldn't affect the contents of the buckets, or be affected by the contents of the buckets.
SIDE NOTE: This may be why we don’t store tubes of toothpaste and tubes of other, very dissimilar contents near each other in the bathroom.
In the end, this approach will work fine with round and square buckets. I posted more details and photos of this on the INSTRUCTABLES site. The web page is a bit more drawn out because you have to have steps to post. The actual page can be seen here:
https://www.instructables.com/Solving-the-Stuck-Stacked-Bucket-Problem-Its-This-/