My existing chain guide and derailleur only went up to about 42t, so I designed this adjustable chain guide for my 48t chain ring that should work with any chain ring size.
You will want to adjust the piece that attaches to your bike as the current one has been customized for my bicycle frame and chain ring position. Zip ties work great for attaching to frames with a flat spot but if you have a round seat post tube you may want to use a pipe clamp.
Tested for about 20km of hard trail riding with no dropped chains.
More info about my electric bike: http://endless-sphere.com/forums/viewtopic.php?f=28&t=50946
Print chain guide.stl, chain guide (mirrored).stl and chain guide2.stl
You can lower the infill (I used 60%) but I recommend increasing perimeters and top/bottom layers to 4-5, as this is a consumable part that will slowly get worn away by the chain.
Take your parts to the hardware store and find some screws that fit.
I believe I used an M4x25 screw + nut for the smaller hole, and two wood screws around 35-40mm long to bolt the 3 pieces together.
Get everything aligned visually then screw it together and strap it to the bike frame with zip ties or pipe clamps.
You can use chain guide (thinner).stl if you need a slimmer chain guide.