Some internet search revealed that you could straighten a warped bridge by steaming over a stove for a long time, clamping back in a straight position, and letting it dry. I called that Plan B. Plan A was to buy a bridge blank and carve it to the right shape.
You can source cured/dried Maple machined bridge blanks from China (Ebay) for next to nothing, I had 4 delivered for $AUD13(2017). The slow boat from China to Australia is around 3 weeks, so you have to plan ahead. I figured 4 should be enough, gives me the chance to break 3 first :-)
First up, to get the feet of the bridge to mate to the face of the cello, I used sandpaper. Place sandpaper on the face, hold the bridge on sandpaper, move slightly back and forward in the area the bridge will stand. This seems very sensible to me, rather than carving the feet to attempt to match the face and constantly checking how you are going.
A leg spreader may or may not be absoltely necessary. I made one out of some bolts and scrap metal with my arc welder and angle grinder. When fitted, the pressure of the strings will push the bridge legs outwards, so pre-spread the legs by around 2mm, sand the feet to mate to the face while spread, then when it's all back together the feet will be a perfect match to the face. I don't think this is really really necessary, I think it is better to do it, but I think you'd be fine without spreading. One of the main reasons I made it is so I could show it to my wife and say "Normally you'd just use a bottle of wine..."
For instance I particularly liked Sally's approach of first getting the feet fitted to the face, installing the bridge straight away although waaay to high, putting the strings roughly in place, measuring the string height at the end of the fretboard, subtracting that from the desired string height, placing a template on the bridge in that position, and cutting off the excess with a jigsaw. Genius.
I fitted the bridge several times, sanded it down some more to get the string heights rights, it was tedious. The last time I took too much off on the high string, I was impatient. Moral there is don't be conservative to start with, because you will get annoyed and be less conservative to end with and stuff it up. Measure twice, cut once. I ended up having to glue a small piece of wood back on. Don't do that.
Another good tip I found on the internet was rather than gluing some parchment on the top where the strings go - or doing nothing at all - just put a dob of superglue there and let it soak in. It dries rock hard, strings slip past it nicely and won't cut into the wood.
I fancied up the look with a knife, took the excess wood out with my die grinder and wrote my initials on it, you can see the difference between the blank and my finished product in this photo. It took me the better part of a day from start to finish, but I learnt a lot. I would imagine if I ever had to make another it would take me around 2 hours.
Another skill added to the list. Don't be afraid to try it if you are in the same predicament as me - and violins, violas, cellos and double basses are all similar. Read Sally's tutorial, watch a few youtube videos, and use some of the hacks I mentioned here. Good luck out there!