Some people prefer stands and other types of trainers over rollers, but to me there are a few appealing aspects of rollers vs others:
Faster / more intense Workouts
More challenging / engaging
Better all-over body workout
No adapting or installation - just set the bike on and go
Some may argue that it isn't worth the cost (a new set can be bought ~ $200) and time to build your own, but that can be said about almost anything-
I enjoy building things, I had all the materials that I needed, and it only took a weekend to build.
The building isn't particularly challenging, so long as you are familiar with basic construction / projecting techniques, and have access to the necessary tools and materials.
I would not however suggest this as a first project, or for someone who does not have experience with similar projects.
P.S. I do not intended for this to be a set of step-by-step construction plans, as I feel that those who will likely have success in completing a project like this will be able to easily fill-in the b_a_ks that I may leave.
P.P.S. This is another homebuilt setup (much more sophisticated than mine) that I didn't find until writing this - however for me the free-motion feature isn't very important :
Step 1: Parts and Materials
I know that there are improvements and tweaks that others will find as they build their own which will result in better and diverse versions.
Some material suggestions:
3" PVC pipe 3pcs @ 18"
8ft of low stretch webbing - or tubing, rope etc
6+ Ball Bearings
Steel Rod 3pcs @ 21"
Dimensional Lumber - or other material for frame
Step 2: Dimensions
The roller width can be as wide - or narrow as you'd like, but unless you have very good balance, I wouldn't recommend less than 14 or so inches. I built mine 18 inches wide.
The distance between centers on most adult bikes are fairly similar - but you may want to build the rollers to the bike you plan on using on it. Making it adjustable is also an option, but didn't seem necessary to me - I have rode four different bikes from full suspension downhill, to road racing on it and no adjustment was necessary
The rear tire will be centered halfway between the rear two rollers, and the center of the front wheel should be slightly behind the center of the front roller. So if your bike is 51.5" center to center, add a couple inches and make it 53.5" or ( (11 / 2) + 38)
Step 3: Rollers
You can also use logs if you like a rougher - more off-road feel
I cut rings of the 3" pvc and used them to create a channel to restrict the webbing from walking around on the roller
Step 4: Bearings
I cut the plywood discs using a jigsaw, and then sanded them to the compass mark
The recess can be made using a paddle bit of forstner bit
I then made retainers out of big fender washers to hold the bearings tightly
If possible, the bearings should be pressed into the discs tightly, if the bit you use doesn't allow for this, epoxy may be necessary to keep them from potentially wearing or slipping in the wood
Step 5: The Frame
I added a little standing platform on both sides just to make it easier to get on and off - but it isn't necessary
2x4 studs or other common materials will work fine - you certainly don't need to use hardwood like is pictured in mine
You will also want a way of pinning the axles so that they don't turn or try working their way out while moving.
Note the notch in the middle cross pieces to accommodate the webbing
Step 6: The Missing Link
I chose to use some 1" webbing, since I had some lying around. Lots of different things could be used for this however, rope, rubber tubing, rubber strapping, etc. The important thing is that you can get it tight, and so that it doesn't slip on the rollers.
It is easiest to figure out how long it needs to be by assembling the frame and rollers, and then measuring and marking directly. It is also better to make it a touch small so that it will be tight when you put it together
The belt is a single piece of webbing - with the two ends overlapped and joined together.
The joint is preferably sewn, but gluing should also work - see comments on pic
Step 7: Putting It All Together
I built mine without the stepping plates first, and then decided to add them afterword
They aren't necessary, but are nice
Some people also like the security of wheels on top to prevent the bike tire from tracking off the edge of the rollers - often seen on the free-motion setups - I didn't feel them necessary
It is nice to assemble the frame with strictly screws or bolts - as you may want to take it apart later to adjust, transport, etc.
Hope you found this helpful - Have fun if you do decide to build your own, and go get some exercise. For both your body and your creativity.
Step 8: Other People's Builds
dhouggy also created an instructable, and those steps are visible here: https://www.instructables.com/id/Bicycle-roller-trainer/
fuj1b1ke's build is shown below in some of the pictures