The boxing type FMs that I have have various densities of dense foam with leather or vinyl on the outside. Some brands (for like Thai Pads) have layers of various densities of the foam. My original Thai pads had fabric like T shirts stuffed in them.
Something to consider is the intended use of them. Like are you going to train bare handed, with boxing gloves, wrapped hands, or even MMA style gloves the majority of the time. I have an old pair of Century FMs that have a really stiff hard core foam in them. They will break your hands almost without gloves on. Like wise I have a pair of KI FMs that have a softer foam core in them that you can work with gloves or bare hands (but preferably gloves). I also have a pair of square hand pads (from Century again) that have a real soft foam core in it that are able to be used for empty hand (even with kids).
Teaching kids I use the soft Century square hand targets but they hit them bare handed with no complaints, if they go with the KIs FMs the older kids (teens) like them but some will still gravitate to the square hand targets since they are easier on the hands. So if you are teaching or working with kids, teens, females, all but some huge guys that like pounding a makawara type surface with unprotected hands, I would recommend that you go with a dense foam but make sure it has some give to it.
Next I would think about how you plan to get them on or off. The square hand targets I have, have two straps on the back that you slip your hand through, easy on, easy off and you don't need help. The KIs have a Velcro strap strap again easy on and off except if you have both FMs on, then your training partner or another third party has to take them off. Something to think about if they are wearing boxing type gloves as they train. The absolute worse idea was a buckle system that my Thai Pads have, what a pain in the you know what to use. If you go with the Velcrostrap make sure it is long enough for people with a big wrist (my KIs are small for all but my teen students) to use.
I had a pair of Thai pads made for me off of my original pair, and it was a pain for all involved. They aren't as good as the originals, the material isn't as good, the quality etc. etc. FMs I think would be even more involved unless they were made on the easy design like the Century square made targets, very simple design, simple to use.
In the end unless you are trying out a new design something pretty custom for your needs or a new idea to market, I'd just save the time and the hassle and buy a pair. Time is money and going around to get the materials, having someone make them etc. etc. to me isn't cost effective when you can order them on line ready made and ready to use.
