Many times the square shoulders on carriage bolts will seat or form the hole as they are tightened in a round hole.
I did tried this approach but my design had two issues that prevent this to work properly, tube thickness is 0.125in and the hole is perfectly tight on 0.5in so, if I try to tight a carriage bolt, the amount of steel that needs to be displaced is significant, it will get in there but will bow a sizeable area, something to think about for the next design.
My approach as of now is to use a leveling mat, like the ones used to lay pavers (light foam) and have cutouts where the bolt heads are, this has an unintended damping effect that is very welcome - if I had more patience today I could have marked each hole precisely and drill a round hole but instead I went for it with an utility knife: