No change to the dimensions.
I have concerns about putting steering under there but it packages so nicely. I guess worst case I could add a printed cover to protect it from the worst.
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What’s the weight?
Onshape is reporting a hair under 4lbs. I’m not sure I buy that one, quick search didn’t find anything wildly under weighted though so I’m not 100% certain where the error is. Using glass filled nylon for the printed parts.
I have a good chunk of one built but need to toss a kitchen scale into my next amazon order to weigh it.
Gut feel says closer to 5lbs than 4lbs after accounting for wire weight.
The only thing I’m still not happy with is mounting it to the rails. Normally would use rivets but rivets are weaker in tension than bolts, so my gut feel is to use bolts with maybe a printed capture for the nuts on the inside of the tube.
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What’s your expected tube thickness & fastener size?
At 0.040 or 0.065 I’d expect failure on the tube face before failure of the fastener, especially if you stepped up to 3/16 rivet. Even 5/32 should be fine on your 16x16 testchassis.
If you’re running bolts, running them all the way through (with a spacer to get some clamping) will do a lot more stiffness than just tacking the module to the top tube face with a nut on the inside. Removes some of the prying moment on the top tube face.
BUT - If you stick with your “one bolt to rule them all”, yeah, you’ll be fine bolting onto the top tube face with a nut held behind it.
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Plan was .100" Versaframe largely because it’s common.
The through the tube with a spacer was my first reaction, it adds a new type of bolt though which I don’t like but I have a hunch it would be more ideal.
Truth, I have a hunch with the 16x16 chassis and likely sub 30lb total weight I could get away with dry angel hair pasta as a structural member. I’ll probably skip on the long bolts and just use rivets with washers in the back to increase surface area against the aluminum.
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Green loctite when installing these helps reduce the chance of Spinning Rivnut Syndrome. SRS is the leading cause of mentor frustration due to student application of rivnuts.
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It also comes with holes on that face, pre-spaced, so you could hacksaw it in your garage and still get a square frame…
Your local scrapyard/bigbox might carry steel with pre-punched holes at some spacing, but you’ll start getting Junkyard Wars vibes 
(It looks like you’re already ordering Vex hex shafts at full length though, so adding some extrusion to the same order won’t blow up your shipping cost)
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Yeah, I’d probably use the horizontal band saw at the local maker space, gotta head down there anyway to do the shafts. Way lower effort.
I’ve considered just slapping it on some 2x1 15 series T-slot, increase in weight but can order it precut. Gets a hair pricey but might be worth it for a test bed since it’ll let me add things onto it easily wherever they need to be.
I actually need to check if I have enough 3/8" hex and 3/8" thunderhex sitting around to finish this without ordering more. That will cut down on shipping costs.
A lesson 1551 learned the hard way that year we attached bumpers with rivnuts…
Having to frantically remove a bumper with an angle grinder (hiding those sparky-sparks from the powers that be) and then putting on the other set with one or more self-tapping screws so we could make our next match was, shall we say, less than ideal.
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Did you cross thread the bolt or gall the threads (dissimilar metals)? Normally, you should have been able to remove the bolt from the rivnut fairly easily.
But as a general rule, I would not use rivnuts for an application where you plan to remove and re-install the bolts numerous times (like attaching bumpers). If you are going to use a threaded attachment like this for your bumpers, it would be better, in my opinion, to use threaded rod to make a stud coming off the frame that the bumpers can slide onto and then a wing nut to secure it.
Also a bit of training on not over-tightening these fasters is a good idea. Using an impact wrench or power drill on the highest torque setting to install your bumpers is just begging for a seizure (of the bolt and your heart)…
Oh, lessons were learned… 
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