Milkjug caps make perfect sabots for 1.5" SCH40. Then use whatever for buckshot. I've launched TONS of rocks and never had anything bad happen... though I make sure the rock can't get wedged in the barrel before I cram it in there.
Any fruit or veggie can be cut to make a slug for any barrel under 2" or so. If I run out of spuds, apples, pears, lemons, even onions (though stinky...) will work fine.
I tend to try to make a bunch of rounds to shoot later. One thing I've frequently done is to take a paper towel tube or toilet paper tube, cut it lengthwise, "roll it" in on itself to reduce the diameter a bit (checking it inside a piece of 1.5" pipe to make sure it's right), then put a wrap or two of duct tape on it to hold the shape. Then fill the tube with whatever you have handy; rocks, garbage, wet sand, etc.... and cap the ends (using cardboard and tape). Make the front end heavier than the back end. I can't say for sure, but I think putting a tail (scrap fabric, or a strip of duct tape folded over onto itself [sticky side to sticky side]) on the back makes it fly straighter.
I'd suggest setting your canon up for interchangeable barrels so you can switch to 2" (caulk tubes...

) or 2.5" (tennis balls) or 1.5" SDR21 (duh).
Peace,
Pete Zaria.