Well, you won't get much resistance if the wheel is just "blowing in the wind" sorta speak. The result would be very little if any workout, and almost no benefit to your body. So, just spinning the wheel isn't really such a good idea.
If you want to build a real trainer, you will need the following...
1- Arc welding machine (to build a steel frame)
1- Centrifugal internal circulating pump (industrial grade)
1- Alloy or synthetic roller of your choice.
1- set of appropriately sized bearings (for the roller)
1- coupling (to attach the roller to the pump)
1- Supply of High grade synthetic oil (for the pump to add resistance)
1- Spring assembly (to provide proper tension)
1- set of pipe nipples and elbows (to recirc oil)
1- throttle valve w/remote actuator (to adjust resistance if you choose)
1- set of various nuts and bolts/screws
1- welding jig (to mount and hold the steel components when welding)
1- welding helmet
1- set of welding leathers
1- slag hammer
1- set of various steel plates in various thicknesses and sizes
1- steel band saw (to cut steel plating for pump mounts and brackets)
1- grinder (stone or angle type, your choice)
1- cooling bucket w/water for hardening small parts
1- 100amp / 220 volt electrical sytem for the welding machine
1- properly ventilated work area
In other words, go buy a trainer.
If you want an affordable unit, Minoura makes less expensive mag trainers that work just fine. Or, if you want one that is smoother and quieter, CycleOps Fluid 2 is rated well for the money. There are tons out there, but the CycleOps is a very common trainer that many people are pleased with. I personally, have a Performance brand TravelTrac with a wireless computer, and couldn't be happier.