Airflow comments:
1.) AIR IS LAZY. It will take the path of least resistance. You see those gaps on the left and right of your screens. They don't matter at this point, but if you fill those trays up, the air is going to look at those gaps and say, "Man, wouldn't it be easier to go there then push my way through all those green things?" And it will. The air will bypass the hops and you'll be drying forever.
2.) Seal that thing up. Close off those vents at the top. Its called "short-circuiting". Basically, the fan will pull air through those instead of the bottom vents and bypass most of the hops. Likewise, use some weather stripping around the door to get a better seal so the air is only coming in the bottom where you want it.
3.) Be ready to shuffle trays. With 5 trays there, the bottoms ones will dry faster than the top. This is because the as the bottom trays give up their moisture to the air, the RH rises. Higher RH means lower driving force to evaporate moisture, so the top hops see a longer drying time. To even this out, shuffle the trays around. Of course this somewhat depends on the velocity of the airflow. If the velocity is high enough it won't matter as much. Of course if it is too high, you will fluidize your hops (make them jiggle/float) and you will shake the lupulin all out of them...assuming this is upflow configuration. What's the CFM of that fan and the area of the cabinet? shoot for around 1 ft/sec through the bed.
This is a good idea, but if you can find it a 35mm film drying cabinet is even better. Obviously, people are getting rid of them these days, they come with fan and heat installed and do a good job of self sealing. Check ebay or craigslist.
50-60 plants? What year are they and how many varieties? If they are all the same (same harvest date), I'm pretty sure you won't have enough capacity in this at full maturity.
