Looking great Martin, you've used a similar diameter exhaust pipe to my Bachmann conversion shown in Davenports on Parade!
I've now added a cap to top using a Brad to keep the rain out - modelled in open position.
By the way if you don't know what they are don't bother Googling as you won't get right answer just mostly the Pitts
If you'll be at Expo NG having just found a box of 100, I'll bring some to Swanley and leave them with Steve. Just ask and he'll supply a couple; he'll be even more pleased if you buy a kit or two
Asking applies to any Gnatterboxer who'd like them.