Often people with mist chambers don't even use the poly bag, as long as the humidity at the graft and scion is high then you're good. A mist chamber would do the trick. You don't don't want the scion to dry out while the graft is healing.

You will still need some natural sunlight so if you can't do outdoors with the mist chamber, use poly bags and find a nice cool shady spot outside somewhere. With summer grafting you aren't going to want ANY direct sun if possible, filtered light or shade only.

If you have the proper lights for growing plants, in the garage at about 65-70 degrees Farenheit and low- medium light for 2 weeks or so, about 8 hours a day would probably work as well. Let us know your results and good luck. I will be trying some myself as well.

The chamber is outside - I have a shade net that would take the light to approx 50% - would that be ok? I usually do not run the mist at nite(8pm-6am) - should I run it while the grafted maple is in there?

Thanks for the advice

I think the shade cloth would be ok if the mist is keeping it moist but I am not sure for I am new to summer grafting. I would keep the mist going 24 hours until the graft heals for drying winds and natural transpiration in the leaves might be too much. Cutting off 3/4 of your scion leaf surface area will help reduce transpiration and raise your chances of a successful union. I am going to try 50% shade cloth as well but only a couple hours of direct sun hits the cloth.