I just took a quick look at a 2.8 Elmar on RitzCollectibles site: it appears that the aperture is set by a knurled ring located behind the front filter mount, therefore the aperture should remain adjustable with a hood installed, unlike to earlier Elmars that had the little tab on the front of the lens... ?
Lately, I have been using a Leitz "barndoor" hood (marked "Summicron") on all my 50mm lenses: S-cron, Summar, and 3.5 Elmar. My little trick for mounting on the A-36 "push-on lenses" is to install the Leitz A-36 to E-39 filter adapter on the lens, then clip the barndoor onto the filter adapter... works pretty well.
I also use the same filter adapter to mount a cast Heavystar hood on my 90 & 135 mm Leitz lenses.
I've been looking for both a FIKUS (50/90/135 telescoping) and the hood that allows you to set the aperture on the front-set Elmars, but they've been running pretyty expensive on evil-bay...
Ritzcollectibles a has a fair selection of Leica Hoods, if you want to go window-shopping...
Luddite Frank