I'm all for fairly large euphonium mouthpieces, but I haven't tried a 1 1/2G. One problem with mouthpieces that large (e.g., the Schilke 60) is that your intonation may suffer severely.
For an easily available and not too expensive mouthpiece, one of the Wicks should be good for you. I'd go with the 3AL (which I often do play on if I'm playing a lot in the low range -- like tuba or bass trombone parts). I think if you look closely at my Avatar here, you'll see that that's a 3AL I'm playing on.
My main euphonium mouthpiece is a Doug Elliott which is very close to the 3AL in size (just slightly smaller) and performance. I have a Wick 2NAL but don't like the sound and response I get from it on a euphonium. With a 3AL you can in fact cover the octave below the bass clef staff if you want to. And it's not too large to make a decent high range straightforwardly playable.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)