I want to do a CMH conversion on a 35w HPS wallpack, and was looking at these cheap ballasts. My question is, would they fit in the fixture? The wallpacks are made by Regent, and they are of the tallpack form factor. Would one of these fit in it?

Oops, here is the link:
http://www.ebay.com/itm/20-Watt-Metal-Halide-Ballast-20W-MH-120V-277V-M2012-27CK-3EU-J-/191685073693?hash=item2ca1521b1d:g:hhMAAOSwd0BVzLAS

I would ask the seller what the dimensions of the ballast are, sounds like a cool idea you have! Sadly 4 out of 5 of my wallpacks are to small for anything except 35 or 70w sodium chokes

I purchased it a few weeks ago, and put it in the wallpack. It fit perectly. I just need to get a g12 CMH lamp and make a holder, and it will be done.

If you use superimposed ignitor (thats what most Pulse Start MH use here, i dont know in the US), the ballast can be remote and wired with ordinary 240V flex. Include only the ignitor in the lantern

But if the lantern is small and made of plastic, it may be just too small for the higher power lamps anyway - It will melt or deteriorate (plastic not visibly changing but getting brittle, ...)

This is what I did. See also the rest of pics in series. Athough you'll have a 20w lamp in yours, try to keep the lamp as far away from the ballast as possible. The vertical orientation of these wallpacks results in plenty of heat above the socket.