If you have a bike shop near you they will have star nuts, but the all thread may be a better choice. I wouldnt think that square tubing would be very expensive, but I could be wrong. Maybe find something to scavenge some tubing from.
I used round 1-1/8 star nuts and used a grinder to square them up. The only problem ive had is not being able to find stainless metric bolts, I may need to order some.