I'd like to attach something, probably my tent, to my bosses on the underside of my downtube. Do I really need anything more than the bow tie strap anchors? Does a cage or something like the DeWidget Strap ‘Deck’ actually help much with stopping the item moving around?

I'd say that it depends on how small and to a degree, how rigid the load is. If it's a squishy mess in a drybag then a cage or deck will certainly help. If it's a tightly packed bundle that isn't too fat then bow-ties will likely work on their own.

I’d try some voile straps backed with some thin flexible plastic, bolts through both into the fork

I've used the Bow Ties under the down tube for a bit over a year and echo Stu's comments. They're perfect for what I fit them for, which is carrying my folding chair (did I mention I carry a chair?) which is rigid enough not to need any other support. A tent might be ok if the poles are in the bag, giving it some structure, but otherwise a deck of some sort would be wise.

I bought a couple these recently. Very cheap, nice and sturdy and come with a pretty good bag. The DeWidget Strap Deck might be a better option though as it doesn't have a base which might restrict how you strap your tent under there.

https://www.planetx.co.uk/i/q/BAPODTFB/ ... e-and-ties

Thanks