Cook pasta according to box directions. Drain and set aside.
In small bowl, whisk the eggs, then set aside.
In small pot over medium-low heat, melt 4 tbsp of butter. Whisk in flour and continue whisking for 2 minutes.
Slowly pour in milk. Whisk until smooth. Let cook until thickened, about 7 minutes. Remove from heat.
Slowly pour ½ cup of sauce into the bowl with the eggs. Whisk constantly while pouring and until smooth. Once smooth, whisk eggs back into pot with remaining mixture. Stir in seasoned salt, and salt & pepper.
In slow cooker, slice remaining 4 tbsp of butter evenly onto bottom of pot. Layer 1/3 of cooked pasta on top of butter inpot and cover with 1/3 of the grated cheddar and 1/3 of the Velveeta cheeses. Continue layering until all of the pasta and cheeses are used.
Pour sauce evenly over top of ingredients in pot.
Cover and cook in slow cooker on low heat for 2 hours, stirring every 30 minutes.