Technically, you can make your own kettlebells -- or more accurately, kettlebell facsimiles. But just because you can craft a kettlebell out of concrete or weight plates doesn't mean you should. Many kettlebell exercises position you -- and the weight -- so that the potential consequences of a homemade-kettlebell blowout are dangerous at best, fatal at worst.
Many kettlebell exercises either position the kettlebell overhead or have you swinging the kettlebell through a partial arc -- sometimes both. Most store-bought kettlebells are cast in one piece, so you don't have to worry about them coming apart and landing on you -- or breaking a window -- as you work out.
Homemade kettlebells, however, are usually cobbled together from several items. So there's a very real risk that said pieces could fall apart and hurt you or someone else when subjected to the forces of a typical workout.
Safer Types of DIY Exercise Equipment
If you're intent upon making your own exercise equipment, consider options for which the stakes aren't quite as high. For example, you can create your own medicine ball by filling a child's basketball with sand, or create your own slosh pipe by filling a PVC pipe half full of water, then capping and sealing the ends.