Very easy....
Before I invested in a nice wheelsmith unit, I made one using a long piece of 1 x 2 oak with a fine threaded bolt running right down the middle as an adjustable axle gauge. The two opposite ends of oak had additional layers of scrap wood glued on to butt up against the outer diameters of the rim. Worked very effectively, I really didn't need to buy one...... but I guess I eventually wanted the real thing!
If I still have this homemade tool laying around somewhere, I'll post photo this evening.