I still use the Bestwell MicroSite. Not very expensive, if you can find one, and works perfectly.
Over the years, I started with a smaller Paterson focus unit. It gave me eye strain. It was a piece of junk; I gave it away. Then, I bought a Bestwell, probably the best combination of quality and cost you'll ever find. In the early 1990s, when all darkroom equipment was being sold off for pennies or junked, I paid $50 for a minty Peak unit. Its only advantage over the Bestwell is that it rotates to focus in the corners of a print rather than just in the center area. So, you can waste your money on a Paterson for $50. Bestwell went out of the business; but someone else picked up making the focusers, about $100. Peak (aka Omega Micro...) where and as found used. New, I think they are still sold in the $250 range. If you are serious about it, it is a once in a lifetime purchase. Treat yourself to a Peak.