IMHO, you can easily modify the Zeagle for SM with minimal investment, so you may end up with SM rig being substantially cheaper than Twins. Sure, price not the main benefit or reason for SM, but it is also a legitimate reason...
What you'll need? Two Cam bands (you can ask in dive clubs or shops for old, scrapped BCs and remove the cam bands, buy new ones, or used ones- it is not expensive), each with a dog clip- you can easily find dozens of sites with photos and examples of how to rig them (try in cavediver.net forums).
About 4 D-rings: two on the shoulder straps, connected to the Bungee loop (using a dog clip, some prefer carabiners etc), another on each side of the waist, where the CAM bands are to be connected.
You can easily DIY some alternative "buttplate" using some webbing and D-rings (again, search for it in cavediver.net- I'll try to post photos of my rig next week) or try something like DSS "Tail plate" (there are also some other similar alternatives used for rebreather bailouts- google for it). any of the mentioned above will pass through the BC crotch harness.
You need to be able to pass the tank valves through the bungee loops (depends on the shape/type of your valves, some not necessarily need to be left/right type!), the cam bands to the "buttplate" (or waist D-rings when the tanks are getting empty). You can skip the buttplate thing and connect directly to the waist D-rings, but it may not be comfortable and it can discourage you to continue with SM...
I am not well acquainted with your BC, but I think you can easily find a way to thread the bungee loop as a single piece of bungee, or use two smaller individual loops.
One more thing you'll probably need to improvise, is something to hold/restrain your wing. As the wing is designed to fold over a single (or doubles, depends on your wing size/shape), when there is no tank on your back it will fold in a most peculiar way. Not only funny, but also entanglement potential and you may find it difficult to drain air from it during the dive if it folds too much. To overcome it, you can use an "No Single Tank Adapter" (NSTA), that is- something that attaches instead of your tanks. I just use a flat, thin aluminium plate about 15" long and 7" width, which weights nothing and its sole purpose is to "flatten" the wing when it is inflated- it looks like a rgular STA, just wider. My buddy made something cooler- he uses the BC cam bands but instead of a tank he puts there half of a PVC sewage pipe with 6" diameter (I think), so the wing folds around it just as with a regular tank..
Regulator setup: most likely you can do well with regular hoses, and there are infinite ways to route them (I'd think each SM diver has its own way to route regulators), so you'll have to play to find what's good for you, and at some point you'll consider getting a right angle adapter.
I think this all you need to begin! Search the web for DIY sidemount rigs and you'll get some pretty ideas how to use your BC. I am quite sure that using existing gear you can build a SM rig with minimal expenses. After all, it is just a piece of bungee, some D-rings, dog clips, cam bands that will allow you to experience and help you decide if you want to buy a dedicated rig, or stay with what you have.
The good thing is that it can also serve as BackMount too, but I can't imagine why you'd want to do such as thing. Usually there is no going back
most likely that the bungee and improvised buttplate can stay also during regular dives so all you have to do is remove the NSTA (or whatever) and put a tank instead...
And, if your buddies are BM, you need to practice with them some air sharing etc, as it is different.