binding barrel bolt meets a carriage bolt

[magicniner]

thanks magicniner. i guess it is a matter of experiment - much appreciated.

i'm currently using mcmaster as a source for stock items (as it allows me to drop in the actual 3d product into fusion 360 - enabling the design) however, their tube selection seems rather marginal.

do you have recommendations for manufacturers for smaller sized metal tubes?

For prototyping buy 1m to 3m lengths of tube with a slightly smaller hole than you think you need. Cut to length yourself with a hacksaw, holding the tube in a vice, use masking tape to mark the length, wrapping tape around it makes it easy to mark a straight 90 degree cut, cut slightly over length and file to size.
You can use a hand drill to open the hole up if required, once you have a definite set of dimensions ask for quotes from machine shops in your country, you're into a minimum 1000 parts so it's viable for professionals to quote.
Avoid getting too caught up in being able to download part models to build your product in CAD, model the tube yourself and have it made once you've prototyped it and have working dimensions.