it vill be hell to klean all those gaps..dont think vify like that or the vet kontroll..stainles steel parts need to be kleaned often..and it have to be easy..so no open gaps..
make dubbel bends. and do it to the inside.and you can bolt it together and make closed corners..mu English is not as good as my steelwork..but you have done very vell ..open a stainless steel Pandora box. (soft iron is just so mutch easyer..)
.this material is hell to vork vith..just to drill holes can be a problem..is it run hot..it hardnent up..and the drill cant brake through the hard shall..
Cleaning stainless is easy; there's several products readily available in the US (Bar Keepers friend for one) or simply use Olive Oil to wipe it down. It won't turn rancid, removes stains and fingerprints rapidly, and restores a simple luster to it. The "gaps" are the result of heat distortion from the tack welds. Whether its mild steel, or stainless, heat will warp it when you use thin sheet metal.
Stainless cutting and drilling needs a good quality fluid. Period. I use Walter Stainless Cut and it will allow my 14" steel cutting blade to slice through a piece of 3/4" stainless rod flawlessly without dulling or heating the blade, or the material. And for drilling, it makes it a snap. Too many people use crap like WD-40 as a cutting fluid because they don't know better, or they're too cheap to use the right tool. Cheap tools and cheap machinists produce crappy results.
As for using mild steel for a range hood, it would very likely rust unless completely coated with a high temp paint. The condensation from cooking, steam, heat cycles, will attack it immediately which makes stainless a far better choice. Dave just needs more experience working with stainless, and some new tricks to help produce the highest quality results he is known for on his bikes. But with the Wife's burgeoning business, I suspect he will be getting plenty of chances to bend that curve
...fuse the two pieces, precision fit will help with that, tack carefully and often,and you want to minimize your contact with air, by using purge gas if you can. The post flow will allow cooling to where it won't sugar, and if possible, (which isn't probably) purge the back side with argon also.
In this application, a back purge is nearly impossible, but, a piece of 2x2x1/4" aluminum angle clamped in position would make a great argon dam, and support the panels, and help extract some heat. I would not have used filler in this application. A large cup, and a 1.5 sec/amp post flow would be my recipe. With a pre-flow of about 2 seconds.
