If you decide to make your own bag (or have someone else sew it for you) buying a one panel voile curtain from Walmart will allow you to make a 2 ply bag. I use my sewing machine and it takes about 15 minutes to get it done when I double or triple stitch the sides. If you don't want to hoist the bag up to drain you can make a simple draining system by doing one of these 2 opions:
1. Buy a use oven grate/tray at a thrift store. Buy a 5 gallon white pain bucket from Walmart for $4. Drill a dozen holes or so in the bottom of the bucket. Place the grate over the top of your keggle and set the BIAB in the paint bucket on top of the grate to drain into the keggle. I do this with most of the 5-6 gallon batches I brew BIAB.
2. Do the same idea as above but instead set the grate on top of a second bucket. Place the drilled bucket on the grate and then move your BIAB grain sack into the drilled bucket to drain or even do a sparge if needed. this requires a bit less coordiantion which is nice if the BIAB sack gets heavy like for an 8 gallon batch.
If you want me to post any pictures of this I would be happy to do so if my reply is unclear. The paint strainer bags will be way too small for your keggle but can be used well in a 5 gallon kettle if you ever want to do a 2.5 gallon stove top brew session indoors or make your own starter wort. I do this for much smaller batches of 1-3 gallons on an electric stove and then ferment in 3.5 gallon food grade buckets you can buy from the walmart bakery or a grocery store for around $1-2. Add in a 50 cent grommet from the hardware store and you are ready to do smaller brews if you want to try a new recipe out. I think the BIAB process is very cool so I don't have to clean my mashtun in the winter.