I would test out some different grains or even another LC before building a hood. What if you went through all the expense/trouble, only to have the same contams?
Picking all the sunnies out of the WBS kinda does sound like a pain. They are tricky enough to grow on coir as micro greens without some molding up in the trays. I have a feeling they buy all the worst batches when used for bird food, so yeah.. I've also heard of bad experiences with it as grain spawn too.
Maybe try out a few bags of brown rice from the same walmart you get the WBS. 5 lb sacks for 3.99, which makes about 7 lb of grain spawn per bag once its cooked, which isn't too bad. Clean, and easy to sterilize like the other smaller grains would be. Thats like $40 for 50lbs buying it like that, but you can find better deals for 10 and 25lb sacks I'm sure. Making it just as cheap as going to the feed store.
Yeah you should still build your own FFU anyway, as much as you are into the hobby now.