But the price seems to have shot up since I got one at a camera fair a few years ago! That Heliopan ring from B&H looks like your best bet if that is indeed a 34mm thread with a standard pitch on the inner ring of the Elmar (Teamwork sell Heliopan in the UK, but can't see that ring on their site). Although SRB don't list 34-39mm, you could ask them to quote you for making one - they do custom stuff at reasonable prices.
You can get at least one device that fits into the accessory shoe of a camera and has a clip on the free end to hold a filter, the join between shoe and filter being flexible. Can't recall what they're called (it's an age thing ).
For both 34mm and 39mm the standard pitch is 0.5mm, and that's what's printed on the Heliopan stepping ring and in the old Hoya catalogue linked above. Leica E39 is definitely 0.5mm pitch, and I've just found a posting on RFF that states the 34mm thread used on some 90mm Elmars is also 0.5mm pitch:
But there are exceptions for some thread diameters, where more than one pitch exists. I've read that the 43mm filter thread on the older version of the Summilux 50 has the less common 0.5mm pitch, but on the other hand I've also found that Hoya 43mm filters (apparently 0.75mm pitch) seem to fit just fine!
39mm is 0.5mm pitch from Leica, Hoya, B+W and everyone else.
46mm is 0.75mm pitch from Leica, Hoya, B+W and everyone else.
So for these sizes and several others (including, as far as I can tell, 34mm/0.5mm), thread pitch simply isn't an issue. I think there are some incompatibilities for filter diameters between 39mm and 46mm, including 43mm and 40.5mm. Leica apparently used a 0.5mm pitch in the earlier E43 Summilux, but 0.75mm is otherwise commoner. B+W at one time made both 0.5mm and 0.75mm versions of their 43mm filters. Hoya apparently only makes 0.75mm, and yet I've tried two of them and they both screw on my E43 Summilux all the way without binding or damage. Make of that what you will!