Here's another link that explains the Omega line of enlargers.
I have a DII and did use a 75mm with the 6 1/2 condenser set. I really did not have that much trouble. Try mounting your 80mm on a flat board, not the small cone that you have now. My 75mm and now my 80mm are mounted on the flat board. Even my 90mm is mounted on a flat board. KHB Photographix does not even list that little cone as an accessory for the DII-D2's. The ligh- fall off may be from the cone, not from the condenser.
I looked for a set a medium format condensers on Ebay. I eventually got a variable condenser kit for about $50. I feel Harry's prices are way, way out of line. The old DII's had a 4 condenser set. Omega eventually replaced the 4 1/2" and 5" with the 4 11/16" one. Check out the link I provided, a couple of years ago when I was looking they still had variable condenser kits.