Craft stores like Michaels carry mold builders latex, casting resin, and catalyst. The latex paints onto objects and can be dried with a hairdryer or overnight. As many layers as you want can be added to make the mold as thick as you want. The mold then peels of the object and can be filed with the resin and catalyst to produce a clear plastic part. This method of casting does not require any risers but it does require a small gate and runner, as well as cores. You will have to hand finish the gate area when the casting is complete but you will not have any parting lines. I wouldn't expect tolerances to be great with this process.....

You could alternatively make most of the frame out of sheet stock with a jigsaw, drill, and sandpaper. Delrin (acetate) is a strong plastic that machines easily and would ge great to use for this.