Cheapest way would be to drill/tap each desired interface to the hole pattern of the UR5 base (or ball locking pin it). Other options:
Adapter plate permanently mounted to the base of the robot and use high load rated toggle clamps to secure the plate. (Con: you would need toggle clamps at every location of mounting)
Base plates manufactured specific for UR5 Vention UR5 Base Mounts (Pros: alittle less expensive, off the shelf, Cons: requires bolts, requires plate at each interface)
There are other fancy ways to do it using electromagnets, vacuum grippers, etc… but these get more and more expensive. I’m eager to see what others in the URcommunity respond with because im sure there are some nifty solutions out there. If you aren’t looking to put a ton effort into developing something I would either do the very first option I said or just spend the money and get a decent quick changer off the shelf to make life easier.