6061 aluminum is a good material to machine and is not horribly expensive like 2024. A 1/4 x 2 x 72 inch bar is about $30.00 and a 1 inch diameter bar of round stock is about $10.00 from MSC. There are many metal suppliers that are much cheaper than MSC but you have to hunt for them in your area. The three rod supports can be made from a length of 8-32 brass threaded rod that can also be purchased from a metal supplier or hardware store. Keep in mind that the largest diameter rod that is going to be supported and machined will probably not exceed 1/8 of an inch so design the hole to clear that (3/16-1/4). There is no need to have the front open as this is a steady rest not a follower rest. If you machine the front open it will require extra work and weaken the part. The rod supports can be located at 90 degrees, 210 degrees and 330 degrees. Since the bottom will be clear (270 degrees) a hole can be drilled and a round shaft inserted and secured with Loctite. The other end of the round shaft can be inserted into the base and also attached with Loctite. The loads on this will be extremely light due to the small size of the parts that will be machined on a WW lathe.