That's a little tricky.
If you wanted to use it just for braising or stewing (bottom heat only), there is a really simple solution. Just get some old tuna fish cans (1 or 2) and rest the DO on top of them and that will give you the clearance to get the coals under the DO.
For baking where you need primarily top down heat, but you have a domed lid, it's a bit tricky.
What you could try is get a cake pan like this (click this link
). Now, use a can opener like the old military style openers or a dremel tool or other cutting wheel and cut the bottom out of the pan. This will give you a metal ring.
Place the ring on top of the dome, and then stack your charcoal inside the ring and on top of the dome lid. The cake pan ring will keep your coals from falling off.
For more stability, get some old wire clothes hangers and then punch two holes in the cake pan ring. Now make some "hooks" that you can attach to the cake pan ring and then fasten those to the dome lid's loop handle. This will keep the ring from sliding off.
The only problem here is that it's going to be really hard to take the lid off and check your dish. So, you'll need to be really sure of your recipe, cooking time, and coal count. Load the coals into your makeshift ring, and then leave it alone till the dish is done.
Alternatively, check eBay and search for "dutch oven lid" and look for a flat style (or one that is flatter than the one you have) that will fit your unit.
Hope that helps.