I've looked at this, but I don't have the money to test it out. Here's the thing. Injection molded cases like those from Pelican and SKB only come in certain, set sizes, and bass guitar gig bags are large, with Mono gig bags being some of the largest of all.
A Mono Vertigo Ultra measures (according to Mono Creators) 49.6" x 18.9" x 5.5" (126.0 cm x 48.0 cm x 14.0 cm).
A Mono Vertigo measures 49.6" x 18.9" x 5.5" (126.0cm x 48.0cm x 14.0cm)
A Mono M80 Classic measures 49.2" x 16.5" x 4.7" (125.0cm x 41.9cm x 11.9cm)
A Mono Stealth measures 50.5" x 17" x 4.7" (128.27cm x 43.8cm x 11.94cm)
A Mono Sleeve measures 50.5" x 17" x 2.5" (128.3cm x 43.2cm x 6.4cm)
A Pelican V800—the widest case they make—measures (internally, according to Pelican) 53.00 x 16.00 x 6.00 in (134.6 x 40.6 x 15.2 cm). Someone here has a thread on
fitting a GruvGear Blade gig bag into one with a bunch of pool noodle foam.
It should be mentioned that the Vault line is a budget line, and not as protective as their standard cases, as evidenced by the fact that the V800 costs only $230 from Amazon, but for our purposes, as we would be putting a heavy duty gig bag inside it, probably works to our advantage.
The other Pelican cases, like the 1750 (13.3" internal width) and the IM3330 (14") are not going to fit any Mono case, and probably won't even fit any other gig bag that has enough padding to actually protect your instrument in transit.
A SKB iSeries 5014-6 measures (internally, according to SKB) 50.5 x 14.5 x 6
This is the size they use for their fitted bass cases, which have only 1/2" padding to either side of the body for a StingRay or Fender bass, so those won't fit any Mono case, either.
An SKB iSeries 4719-8 measures 47 x 19 x 8, and might be long enough on the diagonal to fit a standard M80 Classic, but costs 50% more than the 5014-6.
An SKB iSeries 5616-9 measures 56 x 16 x 9, and might work, but is almost twice the price of the 5014-6.
Now, the obvious observation here is that SKB has the industrial capability to produce iSeries injection molded cases up to 56" long (actually up to 60" long for their longest model, the 6018-8), up to 19" wide, and up to 9" deep, which means they could produce a 5019-6 that would perfectly fit even the biggest Mono Vertigo Ultra bass gig bag, but as I understand it, the tooling to produce new models of injection molded or rotomolded boxes is immense, something like on the order of $500K USD for each model.
And what this really means is that, unless the US military orders a box in that size, neither of these companies is ever going to make one, because there's just not enough of a market in the MI industry to justify the investment solely on that basis.