I've searched high and low for a big XR tank in the quest to build the ultimate overland XR. Unfortunately, I think the African Queens is the only big tank currently on the market, and I wouldn't buy one without seeing it first hand.
However, I've had some other interesting leads: Alex Marr found a 40 Liter alu tank for his XR400 in Paris, I think. I can't dig up the email with the details though. My hunch is that that tank was made by African Queens, but if it is made elsewhere and is priced more reasonably, I'm pretty sure that you could make it work on the 650.
Also, check this out: http://www.horizonsunlimited.com/ubb...ML/000021.html
Apparently, Acerbis used to make a 40L tank for XR 6's long ago. I saw pictures to prove it. I called Acerbis asking for information on this tank and the U.S. reps denie that there ever was such thing. Contact Andy, he sold this tank to a guy in Europe and might know where it is now. I'm trying to get my hands on one too.
As for the weak Acerbis tank mounts, you'll actually find that the rear mount is actually the one that's prone to breakage
I've always expected the front mounts to give out, but even after a couple years of riding my bike on motocross tracks, in Hare Scrambles and on nasty fire roads, they've held up fantastically. But I also don't trust them and here's what I plan to do to rememdy them:
There are three nuts molded into the tank for mounting it on different bikes. If you fabricated a thick, strong plate that bolted into all three nuts, you could extend that plate downwards and have a brace connect that plate to the tabs that come off the frame. The frame tabs should definitely be reinforced. I would think that a very heavy rubber grommet between the plate on the tank and the brace that connects to the frame would be ideal, allowing it to flex just slightly. I don't see any reason why without some experimentation you couldn't create a very robust solution for the front tank mounts.
I'm also curious as to why you don't want to use the Acerbis XR650R rear tank. Will it be in the way of your gear? Sure, it doesn't fit the bike in stock form, but for the price and headache you're going to weather trying to fabricate a new tank, you would probably be better off having the 650L subframe modified to accomodate that rear tank - something that I'll most likely attempt if I take my bike over on my next trip.
Please let me know what you end up doing. Shame that such a fantastic bike has such limited fuel tank options.