I've been trying to figure out the best way to do this for a few weeks
now, and I haven't come up with anything I like yet. I'm working on a
product where 4 pieces of 1/4" round aluminum need to be able to screw
onto four sides of both 1 1/2" OD aluminum tube and 2" OD tube. Both
tubes are .065. Picture a hat rack with the large tube being the
center, but that's not what it is. These parts will be assembled by
the buyer, so it has to be simple, and yes, it does have to be in
separate parts... it can't be welded. Having nuts welded onto the
inside of the tube would probably be best, but the tube is too long to
do that. The best I can come up with is to just send a 4-40 machine
screw all the way through the tube and two pieces of 1/4" at a time,
and put a nut on the opposite side, but the proper lengths for the
screws to do that don't seem to be standard in 4-40 size. It would be
great to figure out a way to screw the rounds directly to each four
sides of the center tube, but I can't think of a way to do it.

Any mechanical geniuses have a good idea?