Last year I bought a secondhand Tod Cutler 16thC Bruegel Messer on myArmoury. After a few modifications, including dyeing the sheath, I set about trying to figure out how to wear it.
The belts in Pieter Bruegel's (the 16th-century Low Country painter whom this sword is named after) paintings are generally narrow (maybe one inch/2.54cm), dark or black, have D-shaped buckles (though spectacle buckles are also occasionally seen) rendered in grey, and no keeper loop. They're apparently worn a bit loose and low for comfort and ease of movement since the belt appears to attach directly to the back of the messer's sheath. The Bruegel messer sheath has a pair of 1-inch slits in the back for this purpose. While possible, there's no indication that the opposite side of the belt is fixed to the clothing or another belt hidden under the outerwear — the parsimonious explanation, supported by Mikko Kuusirati when I raised the subject on myArmoury, is that the friction of the belt against the wearer's clothing is enough to hold it up.
Unfortunately, I worked a little too quickly and in an awkward position on the floor because the work table I usually use is occupied at the moment. This may be why the back wound up a bit sloppy with spots of dye and finish on it.
For the moment, the belt seems to do its job, although I wonder if it would work as well if I weren't wearing a belt with my trousers for it to catch on.