Heres basically how it would work
You reach the Top 50 (only has to be done once month)
You now have an option aside from Manage deck, called "T-50 Deck"
You click on that, and you build your T-50 deck.
This deck is not the deck you play with, but it IS the deck that is used when you are selected in a T-50 battle.
This would make it so that after grinding FG's all day, you don't have to switch to a T-50 deck, and therefor players challenging the t-50 wont be bombarded with stall decks (unless you choose to of course).
At the end of each month, the people that have access to the T-50 deck would be reset, and you have to gain access to it again by reaching T-50 at least once.