While locking collars are pretty cheap and work pretty well, there are other options if you don’t mind getting a spanner out.
You can just use a putlog coupler as-is, or cut the tang off with an angle grinder. If you do up the bolt too tight, it can dent the sleeve (as is the case for my cheap loadable dumbbells)
You can cut a sleeve coupler in half to make two really tightly fitting collars (good for Krok rows, which have a tendency to loosen any normal collar.) The ends can be a bit jagged.
Axle Bars are the cheapest Bars going. Just a piece of Scaffolding or Tube and some collars. You could weld slip-on flanges for bonus style points or just use 48mm collars on both sides of the weights. It will have a random weight, which sucks a bit.
