The original restrictor plates did not lend themselves well to installation over wood/plexiglas without a mounting plate. This was because the pins were straight and only about 1/8" long, thus transmitting any force directly to the plexiglas, which isn't good.
After many requests for this type of installation, I changed the locating pins on the restrictors to a
grooved taper style, and made them longer. These ones work much better now when mounted to wood instead of a mounting plate, but I still wouldn't recommend using restrictors straight into MDF because the holes will elongate in MDF after prolonged use. If you are using a dense particle board or plywood for your control panel, then you should be okay with the restrictors as long as you make the holes for the pins accurate enough. There is a downloadable drilling template on the product page for this.
You can see
http://www.oscarcontrols.com/customize_hotrod.shtml to see how I installed restrictors on a HotRod without a mounting plate. Not long ago, the owner of that HotRod emailed me and said that the restrictors were still working great almost a year later.
EDIT -
I forgot to mention... If you drill the holes in the plexi about 1 size larger than required, then the pins won't be in contact with it, and so eliminating the possibily of cracking it by too much force on the restrictors. This is assuming you are using 1/8" or shallower plexiglas.