Thinner filmholders have also been made by Mido. The original Mido holder was something like a reloadable Quickload with an envelope and was reputed to be tricky to handle, but from what I've read, it may have had more to do with the temperament of the users to put up with a certain degree of fiddliness in exchange for portability than with the design. Mido II holders were like very thin regular filmholders that fit into a clamshell spacer. Midos are not easy to find, but they show up on eBay occasionally, most commonly in 4x5", and they're not cheap. They were also made in 4x10", I believe 5x7", and 8x10". I waited for years to get some in 8x10" where they really provide a huge advantage over regular filmholders in terms of portability.
The original maker, or maybe it was someone associated with the original maker, appeared on eBay himself offering some new Mido holders in odd sizes, and people seemed to think that this indicated the possibility that he would start making them again, but this seems never to have materialized. There was a thread about it on the LF forum a few years ago.