I never found the Mr Limpy stp conversions to be comfortable in any way. Medicine spoon was hard and the funnel was horrible to wear because the funnel makes sure the Limpy doesn't sit against you. Apparently it has worked for some since I found the instructions online, but I sure as hell didn't work for me

(used the Whiz Freedom for that)
Nice thing about the Whiz is that it's quite stiff so it won't colapse on you while peeing. I believe the GoGirl is rather squishy, which makes it more comfortable to wear if you plan on keeping it in your underpants during the day. Also, it does go further back than the Whiz. The first Emisil stp was designed around a GoGirl and the funnel went further back than the Whiz and I think it went as far as I pull a Peecock for peeing.
But if I were you I'd start with the Peecock funnel, if you can't use it as is you might as well haha.