Here's what I ended up building to keep the raccoons out:
Basically I took a 2"x2"x4' length of wood and attached two boards about 8"x4' to the top and bottom, making a sort of "I-beam" and then fixed a 2"x4"x6' to the top board.
To put it on the dumpster you just position the I-beam between the two lids and slide the bottom board of the I-beam under the lids, so that you end up with the 2x2 between the lids and a board above the lids and a board below the lids. Then you just slide it all the way back till the cross beam is over the two lids.
This eliminates the gap between the two lids and still allows the lids to be opened, but as one one large lid rather than two small ones.
With the two lids now moving as one piece the raccoons can't open it because they can't lift the lid while standing on top of it and they can't reach the lid from the ground.
It's working well so far, although the raccoons haven't given up yet...
I'll snap some photo's of the finished piece and add them here when I get a chance.