I recently adopted transfer Lebensohl to better support some invitational sequences. The same issue motivated replacing Smolen with Quest Transfers.
Smolen is great for game forcing 5-4 hands. For invitational hands, Smolen cannot be used and additional sequences are needed.
Quest Transfers provides a solution for both game forcing and invitational hands with 5+♠ and 4♥ or with 5+♥ and 4♠:
3♦ shows invitational or better values with 5+♥ and 4♠
3♥ shows invitational or better values with 5+♠ and 4♥
If opener would not accept the major suit game try, opener will bid three of responder's major (the next step). With hand that can accept the invitation, opener can bid 3NT or 4 of the major.
Quest Transfer are easy to remember. They are similar to Transfer Lebensohl or Jacoby transfers. Quest Transfers don't give up much. Relative to Smolen, you lose the 3♦ continuation and gain the 3♠ continuation. Quest Transfers right side the contract like Smolen does. Quest Transfers come up more often than the Smolen bids they replace since they cover both invitational and game forcing sequences. Additionally, Quest Transfer allows you to play 1NT-2♣-2♦-2♠ as a signoff, providing a negative inference when the bidding goes 1NT-2♣-2♦-2♥.
The biggest downside of the convention is that with 5♥ and 4♠, it gives opponent's a chance to make a lead directing or sacrifice suggesting double of 3♦. It is a bit more dangerous for them to double a Smolen bid.
I found this convention at a PattayaBridge.com, quite a nice bridge website.
Plus... it's free!