I'm going to try this battery (SSB HVT-50D) next, which was recently suggested by Tombie.
60aH and all dimensions same or less than D34.
SSB DryCell HVT-50D AGM VRLA 12V 60Ah 580 CCA - Cyclic Use
This SSB one only came to my attention after I replaced the D34 like-for-like after around 4 years.
It didn't fail but was not holding a charge very well and I didn't want to rely on it for a big trip.
Anyway, hopefully I get a few useful years but it seems about 2 is all I can expect from Optima.
Cheers,
Scott

